From 2e48f82b85017dddd1d5b845cbae776bf3ae19ee Mon Sep 17 00:00:00 2001
From: Matthew James Briggs <matthew.james.briggs@gmail.com>
Date: Tue, 18 Aug 2026 08:31:07 +0000
Subject: [PATCH] feat: expose MusicXML id attributes in mx::api

Beginning with MusicXML 3.1 most elements can carry an id attribute that names
that one element within the document. mx::api dropped nearly all of them. Add an
optional id member to every api type that maps one-to-one onto an element with
an id, and read and write it in mx::impl.

New id fields: NoteData, MeasureData, ClefData, KeyData, TimeChoice,
TransposeData, BarlineData, LyricData, DirectionData, WordsData, SymbolData,
RehearsalData, WedgeStart, WedgeStop, SpannerStart, SpannerStop, PedalLineData,
CurveStart, CurveContinue, CurveStop, TieLetRing, TupletStart, TupletStop,
SoundData and FiguredBassData.

A new IdFunctions.h holds the two impl helpers that read and write the
attribute. ApiCommon.h explains what ids are for and the uniqueness rule that mx
does not enforce.

Left out are the elements where placing the id needs a design decision: the
grouping elements mx::api does not model (direction-type, notations,
articulations, ornaments, technical, beam), the marks folded into MarkData
(dynamics, fermata, accidental-mark), harmony and frame (one DirectionData holds
several ChordData that serialize into a single harmony), credit (both credit and
credit-words carry an id and PageTextData models the pair), and the elements
mx::api does not model at all (print, measure-style, grouping, for-part,
glissando, slide).

synthetic/bracket.3.1.xml and synthetic/dashes.3.1.xml now survive the strict
api round trip and are pinned in roundtrip-baseline.txt.
---
 src/include/mx/api/ApiCommon.h                |  11 +
 src/include/mx/api/BarlineData.h              |   6 +-
 src/include/mx/api/ClefData.h                 |   8 +
 src/include/mx/api/CurveData.h                |  24 +-
 src/include/mx/api/DirectionData.h            |   7 +-
 src/include/mx/api/FiguredBassData.h          |   7 +-
 src/include/mx/api/KeyData.h                  |   9 +-
 src/include/mx/api/LyricData.h                |   9 +-
 src/include/mx/api/MeasureData.h              |   4 +
 src/include/mx/api/NoteData.h                 |  11 +-
 src/include/mx/api/PedalLineData.h            |  11 +-
 src/include/mx/api/RehearsalData.h            |   9 +-
 src/include/mx/api/SoundData.h                |   8 +-
 src/include/mx/api/SpannerData.h              |  15 +-
 src/include/mx/api/SymbolData.h               |   6 +-
 src/include/mx/api/TimeChoice.h               |   6 +
 src/include/mx/api/TransposeData.h            |   7 +
 src/include/mx/api/TupletData.h               |  15 +-
 src/include/mx/api/WedgeData.h                |  15 +-
 src/include/mx/api/WordsData.h                |   9 +-
 src/private/mx/api/TimeChoice.cpp             |   2 +-
 src/private/mx/impl/Converter.cpp             |   5 +-
 src/private/mx/impl/CurveFunctions.h          |  11 +
 src/private/mx/impl/DirectionReader.cpp       |   9 +
 src/private/mx/impl/DirectionWriter.cpp       |  11 +
 src/private/mx/impl/IdFunctions.h             |  37 ++
 src/private/mx/impl/MeasureReader.cpp         |  12 +
 src/private/mx/impl/MeasureWriter.cpp         |   4 +
 src/private/mx/impl/NotationsWriter.cpp       |   3 +
 src/private/mx/impl/NoteFunctions.cpp         |   2 +
 src/private/mx/impl/NoteReader.cpp            |   2 +
 src/private/mx/impl/NoteWriter.cpp            |   3 +
 src/private/mx/impl/PropertiesWriter.cpp      |   5 +
 src/private/mx/impl/SoundFunctions.cpp        |   5 +
 src/private/mx/impl/SpannerFunctions.h        |   8 +-
 src/private/mx/impl/TimeReader.cpp            |   2 +
 src/private/mx/impl/TupletReader.cpp          |   3 +
 src/private/mxtest/api/IdAttributeApiTest.cpp | 429 ++++++++++++++++++
 src/private/mxtest/api/roundtrip-baseline.txt |   5 +
 39 files changed, 729 insertions(+), 26 deletions(-)
 create mode 100644 src/private/mx/impl/IdFunctions.h
 create mode 100644 src/private/mxtest/api/IdAttributeApiTest.cpp

diff --git a/src/include/mx/api/ApiCommon.h b/src/include/mx/api/ApiCommon.h
index 3ff4433aa..6efabd4b0 100644
--- a/src/include/mx/api/ApiCommon.h
+++ b/src/include/mx/api/ApiCommon.h
@@ -32,6 +32,17 @@ constexpr int NUMBER_LEVEL_UNSPECIFIED = -1; // MusicXML 'number' attributes, e.
 constexpr int VALUE_UNSPECIFIED = -1;        // other absent-able ints, e.g. DirectionData::voice
 constexpr Double DOUBLE_UNSPECIFIED = -1.0;  // absent-able doubles, e.g. StaffData::staffSize
 
+// MusicXML lets most elements carry an optional id attribute, a name that identifies that one
+// element within the document. Software uses it to point at a particular note, measure, or
+// marking -- to line playback up with the score, to hang an annotation on a note, or to link one
+// file to another. Every mx::api type that models such an element has an `id` member. Leave it
+// empty and no id attribute is written.
+//
+// An id must be unique within the document and must follow the XML name rules: a letter or an
+// underscore first, then letters, digits, dots, hyphens, or underscores. mx repairs an id that
+// breaks the name rules when it writes the file, but it does not check uniqueness, so an id you
+// invent must not collide with one already in the score.
+
 // Intentional ternary: absent-able bools use Bool::unspecified, not std::optional<bool>.
 // See "mx::api conventions" in AGENTS.md.
 enum class Bool
diff --git a/src/include/mx/api/BarlineData.h b/src/include/mx/api/BarlineData.h
index 967a5112b..05857f979 100644
--- a/src/include/mx/api/BarlineData.h
+++ b/src/include/mx/api/BarlineData.h
@@ -116,10 +116,13 @@ class BarlineData
     RepeatWinged repeatWinged;
     HorizontalAlignment location;
 
+    // The <barline> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     BarlineData()
         : tickTimePosition{0}, barlineType{BarlineType::normal}, ending{}, repeat{false}, repeatTimes{0},
           repeatDirection{RepeatDirection::unspecified}, repeatAfterJump{Bool::unspecified},
-          repeatWinged{RepeatWinged::unspecified}, location{HorizontalAlignment::unspecified}
+          repeatWinged{RepeatWinged::unspecified}, location{HorizontalAlignment::unspecified}, id{}
     {
     }
 };
@@ -134,6 +137,7 @@ MXAPI_EQUALS_MEMBER(repeatDirection)
 MXAPI_EQUALS_MEMBER(repeatAfterJump)
 MXAPI_EQUALS_MEMBER(repeatWinged)
 MXAPI_EQUALS_MEMBER(location)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(BarlineData);
 } // namespace api
diff --git a/src/include/mx/api/ClefData.h b/src/include/mx/api/ClefData.h
index c4feba6cd..81a788a87 100644
--- a/src/include/mx/api/ClefData.h
+++ b/src/include/mx/api/ClefData.h
@@ -6,6 +6,9 @@
 
 #include "mx/api/ApiCommon.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -67,6 +70,10 @@ class ClefData
     // Visibility of the clef via the MusicXML print-object attribute.
     // unspecified -> omit the attribute, yes/no -> write print-object verbatim.
     Bool printObject;
+
+    // The <clef> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     std::string toString() const;
 
     // convenience - set symbol, line and octave for common clefs
@@ -99,6 +106,7 @@ MXAPI_EQUALS_MEMBER(tickTimePosition)
 MXAPI_EQUALS_MEMBER(location)
 MXAPI_EQUALS_MEMBER(additional)
 MXAPI_EQUALS_MEMBER(printObject)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(ClefData);
 } // namespace api
diff --git a/src/include/mx/api/CurveData.h b/src/include/mx/api/CurveData.h
index 8b9670a9d..178919df6 100644
--- a/src/include/mx/api/CurveData.h
+++ b/src/include/mx/api/CurveData.h
@@ -10,6 +10,9 @@
 #include "mx/api/PositionData.h"
 #include "mx/api/SpannerNumber.h"
 
+#include <optional>
+#include <string>
+
 // MusicXML Documentation for Bezier Attributes Group
 // The bezier attribute group is used to indicate the curvature of slurs
 // and ties, representing the control points for a cubic bezier curve.
@@ -100,9 +103,13 @@ struct CurveStart
     bool isColorSpecified;
     ColorData colorData;
 
+    // The id attribute of the element this curve end is written as -- <slur> for a slur,
+    // <tied> for a tie (see ApiCommon.h).
+    std::optional<std::string> id;
+
     CurveStart(CurveType inCurveType)
         : curveType{inCurveType}, number{}, curvePoints{}, curveOrientation{CurveOrientation::unspecified},
-          placement{Placement::unspecified}, lineData{}, isColorSpecified{false}, colorData{}
+          placement{Placement::unspecified}, lineData{}, isColorSpecified{false}, colorData{}, id{}
     {
     }
 };
@@ -119,9 +126,13 @@ struct CurveContinue
     bool isBezierOffset2Specified;
     double bezierOffset2;
 
+    // The id attribute of the element this curve end is written as -- <slur> for a slur,
+    // <tied> for a tie (see ApiCommon.h).
+    std::optional<std::string> id;
+
     CurveContinue(CurveType inCurveType)
         : curveType{inCurveType}, number{}, curvePoints{}, isBezierX2Specified{false}, bezierX2{0.0},
-          isBezierY2Specified{false}, bezierY2{0.0}, isBezierOffset2Specified{false}, bezierOffset2{0.0}
+          isBezierY2Specified{false}, bezierY2{0.0}, isBezierOffset2Specified{false}, bezierOffset2{0.0}, id{}
     {
     }
 };
@@ -132,7 +143,11 @@ struct CurveStop
     SpannerNumber number;
     CurvePoints curvePoints;
 
-    CurveStop(CurveType inCurveType) : curveType{inCurveType}, number{}, curvePoints{}
+    // The id attribute of the element this curve end is written as -- <slur> for a slur,
+    // <tied> for a tie (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    CurveStop(CurveType inCurveType) : curveType{inCurveType}, number{}, curvePoints{}, id{}
     {
     }
 };
@@ -157,6 +172,7 @@ MXAPI_EQUALS_MEMBER(placement)
 MXAPI_EQUALS_MEMBER(lineData)
 MXAPI_EQUALS_MEMBER(isColorSpecified)
 MXAPI_EQUALS_MEMBER(colorData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(CurveStart);
 
@@ -170,6 +186,7 @@ MXAPI_EQUALS_MEMBER(isBezierY2Specified)
 MXAPI_EQUALS_MEMBER(bezierY2)
 MXAPI_EQUALS_MEMBER(isBezierOffset2Specified)
 MXAPI_EQUALS_MEMBER(bezierOffset2)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(CurveContinue);
 
@@ -177,6 +194,7 @@ MXAPI_EQUALS_BEGIN(CurveStop)
 MXAPI_EQUALS_MEMBER(curveType)
 MXAPI_EQUALS_MEMBER(number)
 MXAPI_EQUALS_MEMBER(curvePoints)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(CurveStop);
 } // namespace api
diff --git a/src/include/mx/api/DirectionData.h b/src/include/mx/api/DirectionData.h
index a4f61fae6..6981d2200 100644
--- a/src/include/mx/api/DirectionData.h
+++ b/src/include/mx/api/DirectionData.h
@@ -11,6 +11,7 @@
 #include "mx/api/SoundData.h"
 
 #include <optional>
+#include <string>
 
 namespace mx
 {
@@ -80,10 +81,13 @@ struct DirectionData
     // serialize as their own <figured-bass> elements, not as direction-type content.
     std::vector<FiguredBassData> figuredBasses;
 
+    // The <direction> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     DirectionData()
         : tickTimePosition{0}, placement{Placement::unspecified}, systemRelation{SystemRelation::unspecified}, offset{},
           voice{VALUE_UNSPECIFIED}, isStaffValueSpecified{true}, isSoundDataSpecified{false}, soundData{},
-          directionTypes{}, chords{}, figuredBasses{}
+          directionTypes{}, chords{}, figuredBasses{}, id{}
     {
     }
 };
@@ -106,6 +110,7 @@ MXAPI_EQUALS_MEMBER(soundData)
 MXAPI_EQUALS_MEMBER(directionTypes)
 MXAPI_EQUALS_MEMBER(chords)
 MXAPI_EQUALS_MEMBER(figuredBasses)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(DirectionData);
 } // namespace api
diff --git a/src/include/mx/api/FiguredBassData.h b/src/include/mx/api/FiguredBassData.h
index 26263fbdd..a4ff4d9eb 100644
--- a/src/include/mx/api/FiguredBassData.h
+++ b/src/include/mx/api/FiguredBassData.h
@@ -6,6 +6,7 @@
 
 #include "mx/api/ApiCommon.h"
 
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -48,7 +49,10 @@ class FiguredBassData
     // The optional <duration>, in ticks. A value less than 0 means 'unspecified' (no duration child).
     int durationTimeTicks;
 
-    FiguredBassData() : figures{}, parentheses{Bool::unspecified}, durationTimeTicks{VALUE_UNSPECIFIED}
+    // The <figured-bass> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    FiguredBassData() : figures{}, parentheses{Bool::unspecified}, durationTimeTicks{VALUE_UNSPECIFIED}, id{}
     {
     }
 };
@@ -57,6 +61,7 @@ MXAPI_EQUALS_BEGIN(FiguredBassData)
 MXAPI_EQUALS_MEMBER(figures)
 MXAPI_EQUALS_MEMBER(parentheses)
 MXAPI_EQUALS_MEMBER(durationTimeTicks)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(FiguredBassData);
 
diff --git a/src/include/mx/api/KeyData.h b/src/include/mx/api/KeyData.h
index f4551b371..ff4666ae0 100644
--- a/src/include/mx/api/KeyData.h
+++ b/src/include/mx/api/KeyData.h
@@ -6,6 +6,9 @@
 #include "mx/api/ApiCommon.h"
 #include "mx/api/KeyComponent.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -121,9 +124,12 @@ struct KeyData
     // alterations. When custom is non-empty, then fifths and mode are ignored.
     std::vector<KeyComponent> nonTraditional;
 
+    // The <key> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     KeyData()
         : fifths{0}, cancel{0}, cancelLocation{CancelLocation::unspecified}, mode{KeyMode::unspecified},
-          tickTimePosition{0}, staffIndex{INDEX_UNSPECIFIED}, nonTraditional{}
+          tickTimePosition{0}, staffIndex{INDEX_UNSPECIFIED}, nonTraditional{}, id{}
     {
     }
 };
@@ -136,6 +142,7 @@ MXAPI_EQUALS_MEMBER(mode)
 MXAPI_EQUALS_MEMBER(tickTimePosition)
 MXAPI_EQUALS_MEMBER(staffIndex)
 MXAPI_EQUALS_MEMBER(nonTraditional)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 
 MXAPI_NOT_EQUALS_AND_VECTORS(KeyData);
diff --git a/src/include/mx/api/LyricData.h b/src/include/mx/api/LyricData.h
index f88c6f216..e08993b94 100644
--- a/src/include/mx/api/LyricData.h
+++ b/src/include/mx/api/LyricData.h
@@ -8,6 +8,9 @@
 #include "mx/api/PositionData.h"
 #include "mx/api/PrintData.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -38,7 +41,7 @@ class LyricData
   public:
     LyricData()
         : text{}, verseNumber{}, verseName{}, syllabic{LyricSyllabic::unspecified}, hasExtend{false},
-          extendType{LyricExtendType::unspecified}, positionData{}, printData{}
+          extendType{LyricExtendType::unspecified}, positionData{}, printData{}, id{}
     {
     }
 
@@ -50,6 +53,9 @@ class LyricData
     LyricExtendType extendType;
     PositionData positionData;
     PrintData printData;
+
+    // The <lyric> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
 };
 
 MXAPI_EQUALS_BEGIN(LyricData)
@@ -61,6 +67,7 @@ MXAPI_EQUALS_MEMBER(hasExtend)
 MXAPI_EQUALS_MEMBER(extendType)
 MXAPI_EQUALS_MEMBER(positionData)
 MXAPI_EQUALS_MEMBER(printData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(LyricData);
 } // namespace api
diff --git a/src/include/mx/api/MeasureData.h b/src/include/mx/api/MeasureData.h
index 8a6cde682..6968b6daa 100644
--- a/src/include/mx/api/MeasureData.h
+++ b/src/include/mx/api/MeasureData.h
@@ -111,6 +111,9 @@ class MeasureData
     // before this field existed.
     std::vector<TransposeData> transpositions;
 
+    // The <measure> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     MeasureData()
         : staves{}, timeSignature{}, number{}, measureNumbering{MeasureNumbering::unspecified},
           measureNumberingMultipleRestAlways{Bool::unspecified}, measureNumberingMultipleRestRange{Bool::unspecified},
@@ -141,6 +144,7 @@ MXAPI_EQUALS_MEMBER(keys)
 MXAPI_EQUALS_MEMBER(barlines)
 MXAPI_EQUALS_MEMBER(partSymbol)
 MXAPI_EQUALS_MEMBER(transpositions)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(MeasureData);
 } // namespace api
diff --git a/src/include/mx/api/NoteData.h b/src/include/mx/api/NoteData.h
index 79129b482..d746d18b7 100644
--- a/src/include/mx/api/NoteData.h
+++ b/src/include/mx/api/NoteData.h
@@ -82,7 +82,11 @@ struct TieLetRing
     bool isColorSpecified;
     ColorData colorData;
 
-    TieLetRing() : positionData{}, curveOrientation{CurveOrientation::unspecified}, isColorSpecified{false}, colorData{}
+    // The <tied> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    TieLetRing()
+        : positionData{}, curveOrientation{CurveOrientation::unspecified}, isColorSpecified{false}, colorData{}, id{}
     {
     }
 };
@@ -92,6 +96,7 @@ MXAPI_EQUALS_MEMBER(positionData)
 MXAPI_EQUALS_MEMBER(curveOrientation)
 MXAPI_EQUALS_MEMBER(isColorSpecified)
 MXAPI_EQUALS_MEMBER(colorData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(TieLetRing);
 
@@ -202,6 +207,9 @@ class NoteData
     // similarly when parsing if we see ##misc-data## in this element, it
     // will be parsed into these strings. do not use commas in your misc
     // data strings as these are the delimiter
+
+    // The <note> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
     std::vector<std::string> miscData;
 };
 
@@ -234,6 +242,7 @@ MXAPI_EQUALS_MEMBER(printData)
 MXAPI_EQUALS_MEMBER(noteAttachmentData)
 MXAPI_EQUALS_MEMBER(lyrics)
 MXAPI_EQUALS_MEMBER(miscData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(NoteData);
 } // namespace api
diff --git a/src/include/mx/api/PedalLineData.h b/src/include/mx/api/PedalLineData.h
index 53c5c6c64..8606bcfc9 100644
--- a/src/include/mx/api/PedalLineData.h
+++ b/src/include/mx/api/PedalLineData.h
@@ -7,6 +7,9 @@
 #include "mx/api/ApiCommon.h"
 #include "mx/api/PositionData.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -50,11 +53,14 @@ struct PedalLineData
     int tickTimePosition;
     PositionData positionData;
 
-    PedalLineData() : kind{PedalLineKind::unspecified}, tickTimePosition{0}, positionData{}
+    // The <pedal> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    PedalLineData() : kind{PedalLineKind::unspecified}, tickTimePosition{0}, positionData{}, id{}
     {
     }
 
-    PedalLineData(PedalLineKind inKind) : kind{inKind}, tickTimePosition{0}, positionData{}
+    PedalLineData(PedalLineKind inKind) : kind{inKind}, tickTimePosition{0}, positionData{}, id{}
     {
     }
 };
@@ -63,6 +69,7 @@ MXAPI_EQUALS_BEGIN(PedalLineData)
 MXAPI_EQUALS_MEMBER(kind)
 MXAPI_EQUALS_MEMBER(tickTimePosition)
 MXAPI_EQUALS_MEMBER(positionData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(PedalLineData);
 } // namespace api
diff --git a/src/include/mx/api/RehearsalData.h b/src/include/mx/api/RehearsalData.h
index 73685bcfd..a34430155 100644
--- a/src/include/mx/api/RehearsalData.h
+++ b/src/include/mx/api/RehearsalData.h
@@ -9,6 +9,9 @@
 #include "mx/api/FontData.h"
 #include "mx/api/PositionData.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -32,9 +35,12 @@ class RehearsalData
     // and says which edge of the text the position refers to; MusicXML defines both.
     HorizontalAlignment justify;
 
+    // The <rehearsal> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     RehearsalData()
         : text{}, positionData{}, isColorSpecified{false}, colorData{}, fontData{}, enclosure{Enclosure::unspecified},
-          justify{HorizontalAlignment::unspecified}
+          justify{HorizontalAlignment::unspecified}, id{}
     {
     }
 };
@@ -47,6 +53,7 @@ MXAPI_EQUALS_MEMBER(colorData)
 MXAPI_EQUALS_MEMBER(fontData)
 MXAPI_EQUALS_MEMBER(enclosure)
 MXAPI_EQUALS_MEMBER(justify)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(RehearsalData);
 } // namespace api
diff --git a/src/include/mx/api/SoundData.h b/src/include/mx/api/SoundData.h
index 010080e37..121a12999 100644
--- a/src/include/mx/api/SoundData.h
+++ b/src/include/mx/api/SoundData.h
@@ -87,10 +87,13 @@ struct SoundData
 
     std::optional<SwingData> swing;
 
+    // The <sound> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     SoundData()
         : tempo{DOUBLE_UNSPECIFIED}, dynamics{DOUBLE_UNSPECIFIED}, dacapo{Bool::unspecified},
           forwardRepeat{Bool::unspecified}, pizzicato{Bool::unspecified}, segno{}, dalsegno{}, coda{}, tocoda{}, fine{},
-          swing{}
+          swing{}, id{}
     {
     }
 
@@ -98,7 +101,7 @@ struct SoundData
     {
         return tempo >= 0.0 || dynamics >= 0.0 || dacapo != Bool::unspecified || forwardRepeat != Bool::unspecified ||
                pizzicato != Bool::unspecified || !segno.empty() || !dalsegno.empty() || !coda.empty() ||
-               !tocoda.empty() || !fine.empty() || swing.has_value();
+               !tocoda.empty() || !fine.empty() || swing.has_value() || id.has_value();
     }
 };
 
@@ -123,6 +126,7 @@ MXAPI_EQUALS_MEMBER(coda)
 MXAPI_EQUALS_MEMBER(tocoda)
 MXAPI_EQUALS_MEMBER(fine)
 MXAPI_EQUALS_MEMBER(swing)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(SoundData);
 } // namespace api
diff --git a/src/include/mx/api/SpannerData.h b/src/include/mx/api/SpannerData.h
index 51645990f..dd7c55d8a 100644
--- a/src/include/mx/api/SpannerData.h
+++ b/src/include/mx/api/SpannerData.h
@@ -10,6 +10,7 @@
 #include "mx/api/PrintData.h"
 #include "mx/api/SpannerNumber.h"
 
+#include <optional>
 #include <string>
 
 namespace mx
@@ -25,7 +26,11 @@ struct SpannerStart
     PrintData printData;
     LineData lineData;
 
-    SpannerStart() : number{}, tickTimePosition{0}, positionData{}, printData{}, lineData{}
+    // The id attribute of the element this spanner end is written as -- <bracket>,
+    // <dashes>, or <octave-shift> (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    SpannerStart() : number{}, tickTimePosition{0}, positionData{}, printData{}, lineData{}, id{}
     {
     }
 };
@@ -37,7 +42,11 @@ struct SpannerStop
     PositionData positionData;
     LineData lineData;
 
-    SpannerStop() : number{}, tickTimePosition{0}, positionData{}, lineData{}
+    // The id attribute of the element this spanner end is written as -- <bracket>,
+    // <dashes>, or <octave-shift> (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    SpannerStop() : number{}, tickTimePosition{0}, positionData{}, lineData{}, id{}
     {
     }
 };
@@ -48,6 +57,7 @@ MXAPI_EQUALS_MEMBER(tickTimePosition)
 MXAPI_EQUALS_MEMBER(positionData)
 MXAPI_EQUALS_MEMBER(printData)
 MXAPI_EQUALS_MEMBER(lineData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(SpannerStart);
 
@@ -56,6 +66,7 @@ MXAPI_EQUALS_MEMBER(number)
 MXAPI_EQUALS_MEMBER(tickTimePosition)
 MXAPI_EQUALS_MEMBER(positionData)
 MXAPI_EQUALS_MEMBER(lineData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(SpannerStop);
 } // namespace api
diff --git a/src/include/mx/api/SymbolData.h b/src/include/mx/api/SymbolData.h
index 8b6d88742..3b215534b 100644
--- a/src/include/mx/api/SymbolData.h
+++ b/src/include/mx/api/SymbolData.h
@@ -41,9 +41,12 @@ class SymbolData
     // symbol-formatting attributes mirror the text-formatting ones on `<words>`.
     HorizontalAlignment justify;
 
+    // The <symbol> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     SymbolData()
         : smufl{}, positionData{}, fontData{}, color{}, enclosure{Enclosure::unspecified},
-          justify{HorizontalAlignment::unspecified}
+          justify{HorizontalAlignment::unspecified}, id{}
     {
     }
 };
@@ -55,6 +58,7 @@ MXAPI_EQUALS_MEMBER(fontData)
 MXAPI_EQUALS_MEMBER(color)
 MXAPI_EQUALS_MEMBER(enclosure)
 MXAPI_EQUALS_MEMBER(justify)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(SymbolData);
 } // namespace api
diff --git a/src/include/mx/api/TimeChoice.h b/src/include/mx/api/TimeChoice.h
index a16a3fcb1..d26ba44e2 100644
--- a/src/include/mx/api/TimeChoice.h
+++ b/src/include/mx/api/TimeChoice.h
@@ -8,6 +8,8 @@
 #include "mx/api/ComplexTimeSignature.h"
 #include "mx/api/TimeSignatureData.h"
 
+#include <optional>
+#include <string>
 #include <variant>
 
 namespace mx
@@ -57,6 +59,10 @@ class TimeChoice
     // Print/hide the time signature (<time print-object=...>). Ignored when isImplicit.
     Bool display{Bool::unspecified};
 
+    // The <time> element's id attribute (see ApiCommon.h). Only the measure that states the time
+    // signature carries it; a measure that inherits the signature (isImplicit) has no id.
+    std::optional<std::string> id;
+
     // Defaults to a simple, implicit 4/4.
     TimeChoice();
 
diff --git a/src/include/mx/api/TransposeData.h b/src/include/mx/api/TransposeData.h
index d5c55a99b..ac678af64 100644
--- a/src/include/mx/api/TransposeData.h
+++ b/src/include/mx/api/TransposeData.h
@@ -6,6 +6,9 @@
 
 #include "mx/api/ApiCommon.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -70,6 +73,9 @@ class TransposeData
     /// Supports a transposition change somewhere other than at the start of a measure.
     int tickTimePosition = 0;
 
+    /// The <transpose> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     /// If both chromatic and diatonic are zero, then TransposeData is unused (i.e. it need
     /// not be encoded in the MusicXML output).
     inline bool isUsed() const
@@ -83,6 +89,7 @@ MXAPI_EQUALS_MEMBER(chromatic)
 MXAPI_EQUALS_MEMBER(diatonic)
 MXAPI_EQUALS_MEMBER(staffIndex)
 MXAPI_EQUALS_MEMBER(tickTimePosition)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(TransposeData);
 } // namespace api
diff --git a/src/include/mx/api/TupletData.h b/src/include/mx/api/TupletData.h
index 3991f1106..7868c564f 100644
--- a/src/include/mx/api/TupletData.h
+++ b/src/include/mx/api/TupletData.h
@@ -9,6 +9,9 @@
 #include "mx/api/LineData.h"
 #include "mx/api/PositionData.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -46,12 +49,15 @@ class TupletStart
 
     Bool bracket;
 
+    // The <tuplet> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     TupletStart()
         : numberLevel{NUMBER_LEVEL_UNSPECIFIED}, positionData{}, actualNumber{VALUE_UNSPECIFIED},
           actualDurationName{api::DurationName::unspecified}, actualDots{VALUE_UNSPECIFIED},
           normalNumber{VALUE_UNSPECIFIED}, normalDurationName{api::DurationName::unspecified},
           normalDots{VALUE_UNSPECIFIED}, showActualNumber{Bool::unspecified}, showNormalNumber{Bool::unspecified},
-          bracket{Bool::unspecified}
+          bracket{Bool::unspecified}, id{}
     {
     }
 };
@@ -65,7 +71,10 @@ class TupletStop
 
     PositionData positionData;
 
-    TupletStop() : numberLevel{NUMBER_LEVEL_UNSPECIFIED}, positionData{}
+    // The <tuplet> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    TupletStop() : numberLevel{NUMBER_LEVEL_UNSPECIFIED}, positionData{}, id{}
     {
     }
 };
@@ -82,12 +91,14 @@ MXAPI_EQUALS_MEMBER(normalDots)
 MXAPI_EQUALS_MEMBER(showActualNumber)
 MXAPI_EQUALS_MEMBER(showNormalNumber)
 MXAPI_EQUALS_MEMBER(bracket)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(TupletStart);
 
 MXAPI_EQUALS_BEGIN(TupletStop)
 MXAPI_EQUALS_MEMBER(numberLevel)
 MXAPI_EQUALS_MEMBER(positionData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(TupletStop);
 } // namespace api
diff --git a/src/include/mx/api/WedgeData.h b/src/include/mx/api/WedgeData.h
index a307e0f6f..505a465d8 100644
--- a/src/include/mx/api/WedgeData.h
+++ b/src/include/mx/api/WedgeData.h
@@ -10,6 +10,9 @@
 #include "mx/api/PositionData.h"
 #include "mx/api/SpannerNumber.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -32,9 +35,12 @@ struct WedgeStart
     bool isColorSpecified;
     ColorData colorData;
 
+    // The <wedge> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     WedgeStart()
         : number{}, wedgeType{WedgeType::unspecified}, isSpreadSpecified{false}, spread{0.0}, lineData{},
-          positionData{}, isColorSpecified{false}, colorData{}
+          positionData{}, isColorSpecified{false}, colorData{}, id{}
     {
     }
 };
@@ -46,7 +52,10 @@ struct WedgeStop
     bool isSpreadSpecified;
     double spread;
 
-    WedgeStop() : number{}, positionData{}, isSpreadSpecified{false}, spread{0.0}
+    // The <wedge> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
+    WedgeStop() : number{}, positionData{}, isSpreadSpecified{false}, spread{0.0}, id{}
     {
     }
 };
@@ -60,6 +69,7 @@ MXAPI_EQUALS_MEMBER(lineData)
 MXAPI_EQUALS_MEMBER(positionData)
 MXAPI_EQUALS_MEMBER(isColorSpecified)
 MXAPI_EQUALS_MEMBER(colorData)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(WedgeStart);
 
@@ -68,6 +78,7 @@ MXAPI_EQUALS_MEMBER(number)
 MXAPI_EQUALS_MEMBER(positionData)
 MXAPI_EQUALS_MEMBER(isSpreadSpecified)
 MXAPI_EQUALS_MEMBER(spread)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(WedgeStop);
 } // namespace api
diff --git a/src/include/mx/api/WordsData.h b/src/include/mx/api/WordsData.h
index 80e558f38..11fb1a096 100644
--- a/src/include/mx/api/WordsData.h
+++ b/src/include/mx/api/WordsData.h
@@ -9,6 +9,9 @@
 #include "mx/api/FontData.h"
 #include "mx/api/PositionData.h"
 
+#include <optional>
+#include <string>
+
 namespace mx
 {
 namespace api
@@ -35,9 +38,12 @@ class WordsData
     // block against its anchor point; MusicXML defines both attributes on `<words>`.
     HorizontalAlignment justify;
 
+    // The <words> element's id attribute (see ApiCommon.h).
+    std::optional<std::string> id;
+
     WordsData()
         : text{}, positionData{}, fontData{}, isColorSpecified{false}, colorData{}, enclosure{Enclosure::unspecified},
-          justify{HorizontalAlignment::unspecified}
+          justify{HorizontalAlignment::unspecified}, id{}
     {
     }
 };
@@ -50,6 +56,7 @@ MXAPI_EQUALS_MEMBER(isColorSpecified)
 MXAPI_EQUALS_MEMBER(colorData)
 MXAPI_EQUALS_MEMBER(enclosure)
 MXAPI_EQUALS_MEMBER(justify)
+MXAPI_EQUALS_MEMBER(id)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(WordsData);
 } // namespace api
diff --git a/src/private/mx/api/TimeChoice.cpp b/src/private/mx/api/TimeChoice.cpp
index 789e012e3..ac4c2491f 100644
--- a/src/private/mx/api/TimeChoice.cpp
+++ b/src/private/mx/api/TimeChoice.cpp
@@ -111,7 +111,7 @@ const ComplexTimeSignature TimeChoice::complex() const
 
 bool TimeChoice::operator==(const TimeChoice &other) const
 {
-    return isImplicit == other.isImplicit && display == other.display && myValue == other.myValue;
+    return isImplicit == other.isImplicit && display == other.display && id == other.id && myValue == other.myValue;
 }
 
 } // namespace api
diff --git a/src/private/mx/impl/Converter.cpp b/src/private/mx/impl/Converter.cpp
index 8dc0b8709..f6e847fb4 100644
--- a/src/private/mx/impl/Converter.cpp
+++ b/src/private/mx/impl/Converter.cpp
@@ -6,6 +6,7 @@
 #include "mx/core/Decimal.h"
 #include "mx/core/generated/Semitones.h"
 #include "mx/core/generated/TransposeGroup.h"
+#include "mx/impl/IdFunctions.h"
 
 #include <algorithm>
 #include <cmath>
@@ -2323,7 +2324,9 @@ mx::api::TransposeData Converter::convertToTransposeData(const mx::core::Transpo
     const auto octaves = specifiedOctave.has_value() ? specifiedOctave.value() : 0;
     diatonic += octaves * 7;
     const auto chromatic = specifiedChromatic + (octaves * 12);
-    return mx::api::TransposeData{chromatic, diatonic};
+    mx::api::TransposeData out{chromatic, diatonic};
+    out.id = getId(inTranspose);
+    return out;
 }
 } // namespace impl
 } // namespace mx
diff --git a/src/private/mx/impl/CurveFunctions.h b/src/private/mx/impl/CurveFunctions.h
index 46992d7dc..26891aeda 100644
--- a/src/private/mx/impl/CurveFunctions.h
+++ b/src/private/mx/impl/CurveFunctions.h
@@ -8,6 +8,7 @@
 #include "mx/api/NoteData.h"
 #include "mx/core/generated/Slur.h"
 #include "mx/core/generated/Tied.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/LineFunctions.h"
 #include "mx/impl/PositionFunctions.h"
 #include "mx/impl/PrintFunctions.h"
@@ -105,6 +106,8 @@ api::CurveStart parseCurveStart(const SLUR_OR_TIE_ELEMENT_TYPE &inSlurOrTie)
         c.curveOrientation = *inAttributes.orientation() == core::OverUnder::over() ? api::CurveOrientation::overhand
                                                                                     : api::CurveOrientation::underhand;
     }
+
+    c.id = getId(inAttributes);
     return c;
 }
 
@@ -139,6 +142,7 @@ api::CurveContinue parseCurveContinue(const SLUR_OR_TIE_ELEMENT_TYPE &inSlurOrTi
     }
 
     c.curvePoints = parseCurvePoints(inAttributes);
+    c.id = getId(inAttributes);
     return c;
 }
 
@@ -154,6 +158,7 @@ template <typename SLUR_OR_TIE_ELEMENT_TYPE> api::CurveStop parseCurveStop(const
     }
 
     c.curvePoints = parseCurvePoints(inAttributes);
+    c.id = getId(inAttributes);
     return c;
 }
 
@@ -167,6 +172,7 @@ void writeAttributesFromCurveStart(const api::CurveStart inCurve, ATTRIBUTES_TYP
 {
     using CurveTypeAttribute = std::decay_t<decltype(outAttributes.type())>;
     outAttributes.setType(CurveTypeAttribute::start());
+    impl::setId(inCurve.id, outAttributes);
     impl::setAttributesFromPositionData(inCurve.curvePoints.positionData, outAttributes);
     impl::setAttributesFromLineData(inCurve.lineData, outAttributes);
 
@@ -210,6 +216,7 @@ void writeAttributesFromCurveContinue(const api::CurveContinue inCurve, ATTRIBUT
 {
     using CurveTypeAttribute = std::decay_t<decltype(outAttributes.type())>;
     outAttributes.setType(CurveTypeAttribute::continue_());
+    impl::setId(inCurve.id, outAttributes);
     impl::setAttributesFromPositionData(inCurve.curvePoints.positionData, outAttributes);
 
     if (inResolvedNumber.has_value())
@@ -255,6 +262,7 @@ void writeAttributesFromCurveStop(const api::CurveStop inCurve, ATTRIBUTES_TYPE
 {
     using CurveTypeAttribute = std::decay_t<decltype(outAttributes.type())>;
     outAttributes.setType(CurveTypeAttribute::stop());
+    impl::setId(inCurve.id, outAttributes);
     impl::setAttributesFromPositionData(inCurve.curvePoints.positionData, outAttributes);
 
     if (inResolvedNumber.has_value())
@@ -285,6 +293,7 @@ inline void writeAttributesFromTieLetRing(const api::TieLetRing &inTie, core::Ti
 {
     outTied.setType(core::TiedType::letRing());
     impl::setAttributesFromPositionData(inTie.positionData, outTied);
+    impl::setId(inTie.id, outTied);
 
     if (inTie.isColorSpecified)
     {
@@ -318,6 +327,8 @@ inline api::TieLetRing parseTieLetRing(const core::Tied &inTied)
         c.curveOrientation = *inTied.orientation() == core::OverUnder::over() ? api::CurveOrientation::overhand
                                                                               : api::CurveOrientation::underhand;
     }
+
+    c.id = getId(inTied);
     return c;
 }
 
diff --git a/src/private/mx/impl/DirectionReader.cpp b/src/private/mx/impl/DirectionReader.cpp
index 8399af94c..b24135e33 100644
--- a/src/private/mx/impl/DirectionReader.cpp
+++ b/src/private/mx/impl/DirectionReader.cpp
@@ -84,6 +84,7 @@
 #include "mx/core/generated/WedgeType.h"
 #include "mx/core/generated/YesNo.h"
 #include "mx/impl/DynamicsReader.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/MarkDataFunctions.h"
 #include "mx/impl/MetronomeReader.h"
 #include "mx/impl/PrintFunctions.h"
@@ -125,6 +126,7 @@ mx::api::DirectionData DirectionReader::initializeData()
     if (myDirection)
     {
         result.isStaffValueSpecified = myDirection->staff().has_value();
+        result.id = getId(*myDirection);
     }
     else if (myHarmony)
     {
@@ -367,6 +369,7 @@ void DirectionReader::parseRehearsal(const core::DirectionType &directionType)
         {
             outRehearsal.justify = myConverter.convert(*rehearsal.justify());
         }
+        outRehearsal.id = getId(rehearsal);
         myOutDirectionData.directionTypes.emplace_back(api::DirectionChoice{std::move(outRehearsal)});
     }
 }
@@ -428,6 +431,7 @@ void DirectionReader::parseWordsRun(const core::DirectionType &directionType)
             {
                 outSymbol.justify = myConverter.convert(*symbolEl.justify());
             }
+            outSymbol.id = getId(symbolEl);
             run.emplace_back(std::move(outSymbol));
             continue;
         }
@@ -449,6 +453,7 @@ void DirectionReader::parseWordsRun(const core::DirectionType &directionType)
         {
             outWords.justify = myConverter.convert(*wordEl.justify());
         }
+        outWords.id = getId(wordEl);
         run.emplace_back(std::move(outWords));
     }
 
@@ -510,6 +515,7 @@ void DirectionReader::parseWedge(const core::DirectionType &directionType)
             stop.spread = spread;
         }
         stop.positionData = positionData;
+        stop.id = getId(wedge);
         myOutDirectionData.directionTypes.emplace_back(api::DirectionChoice{std::move(stop)});
         return;
     }
@@ -530,6 +536,7 @@ void DirectionReader::parseWedge(const core::DirectionType &directionType)
         start.lineData = lineData;
         start.isColorSpecified = isColorSpecified;
         start.colorData = colorData;
+        start.id = getId(wedge);
         myOutDirectionData.directionTypes.emplace_back(api::DirectionChoice{std::move(start)});
     }
 }
@@ -622,6 +629,7 @@ void DirectionReader::parseBracket(const core::DirectionType &directionType)
         start.positionData = this->parsePositionData(bracket);
         start.lineData = makeBracketLineData();
         start.printData = impl::getPrintData(bracket);
+        start.id = getId(bracket);
         myOutDirectionData.directionTypes.emplace_back(api::DirectionChoice::bracketStart(std::move(start)));
         return;
     }
@@ -677,6 +685,7 @@ void DirectionReader::parsePedal(const core::DirectionType &directionType)
         pedalData.tickTimePosition = myOutDirectionData.tickTimePosition;
         pedalData.positionData = getPositionData(pedal);
         pedalData.positionData.placement = placement;
+        pedalData.id = getId(pedal);
         myOutDirectionData.directionTypes.emplace_back(api::DirectionChoice{std::move(pedalData)});
         return;
     }
diff --git a/src/private/mx/impl/DirectionWriter.cpp b/src/private/mx/impl/DirectionWriter.cpp
index a861a3bec..dfeb9fefb 100644
--- a/src/private/mx/impl/DirectionWriter.cpp
+++ b/src/private/mx/impl/DirectionWriter.cpp
@@ -109,6 +109,7 @@
 #include "mx/core/generated/YesNo.h"
 #include "mx/impl/DynamicsWriter.h"
 #include "mx/impl/FontFunctions.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/LineFunctions.h"
 #include "mx/impl/MarkDataFunctions.h"
 #include "mx/impl/PrintFunctions.h"
@@ -165,6 +166,7 @@ std::vector<core::MusicDataChoice> DirectionWriter::getDirectionLikeThings()
     // nullopt for unspecified, and for the bottom-of-system values that only measure numbering has.
     // <harmony> carries the same attribute; createHarmonyElements writes it there too.
     direction.setSystem(myConverter.convertDirectionSystemRelation(myDirectionData.systemRelation));
+    setId(myDirectionData.id, direction);
 
     if (myDirectionData.isStaffValueSpecified || myCursor.staffIndex != 0)
     {
@@ -331,6 +333,7 @@ void DirectionWriter::emitPedal(const api::PedalLineData &item, core::Direction
     pedal.setType(corePedalType(item.kind));
     pedal.setLine(core::YesNo::yes());
     setAttributesFromPositionData(item.positionData, pedal);
+    setId(item.id, pedal);
     core::DirectionType dt{};
     dt.setChoice(core::DirectionTypeChoice::pedal(pedal));
     addDirectionType(std::move(dt), direction);
@@ -352,6 +355,7 @@ void DirectionWriter::emitWedgeStop(const api::WedgeStop &wedgeStop, const void
         wedge.setSpread(core::Tenths{core::Decimal{static_cast<double>(wedgeStop.spread)}});
     }
     setAttributesFromPositionData(wedgeStop.positionData, wedge);
+    setId(wedgeStop.id, wedge);
     core::DirectionType dt{};
     dt.setChoice(core::DirectionTypeChoice::wedge(wedge));
     addDirectionType(std::move(dt), direction);
@@ -384,6 +388,7 @@ void DirectionWriter::emitWedgeStart(const api::WedgeStart &wedgeStart, const vo
     {
         setAttributesFromColorData(wedgeStart.colorData, wedge);
     }
+    setId(wedgeStart.id, wedge);
     core::DirectionType dt{};
     dt.setChoice(core::DirectionTypeChoice::wedge(wedge));
     addDirectionType(std::move(dt), direction);
@@ -409,6 +414,7 @@ void DirectionWriter::emitOttavaStart(const api::OttavaStart &ottavaStart, const
     impl::setAttributesFromPositionData(ottavaStart.spannerStart.positionData, os);
     impl::setAttributesFromPrintData(ottavaStart.spannerStart.printData, os);
     impl::setAttributesFromLineData(ottavaStart.spannerStart.lineData, os);
+    impl::setId(ottavaStart.spannerStart.id, os);
 
     const auto number = myNumberResolver.emittedNumber(ottavaStart.spannerStart.number, inIdentity);
     if (number.has_value())
@@ -749,6 +755,7 @@ void DirectionWriter::emitWordsRun(const std::vector<api::WordsChoice> &inRun, c
             {
                 outSymbol.setJustify(myConverter.convert(symbolData.justify));
             }
+            setId(symbolData.id, outSymbol);
             choiceItem = core::DirectionTypeChoiceChoice::symbol(std::move(outSymbol));
         }
         else
@@ -770,6 +777,7 @@ void DirectionWriter::emitWordsRun(const std::vector<api::WordsChoice> &inRun, c
             {
                 outWords.setJustify(myConverter.convert(wordsData.justify));
             }
+            setId(wordsData.id, outWords);
             choiceItem = core::DirectionTypeChoiceChoice::words(std::move(outWords));
         }
 
@@ -851,6 +859,7 @@ void DirectionWriter::emitRehearsal(const api::RehearsalData &item, core::Direct
     {
         rehearsal.setJustify(myConverter.convert(item.justify));
     }
+    setId(item.id, rehearsal);
     core::DirectionType dt{};
     dt.setChoice(core::DirectionTypeChoice::rehearsal(core::OneOrMore<core::FormattedTextID>{std::move(rehearsal)}));
     addDirectionType(std::move(dt), direction);
@@ -1812,6 +1821,8 @@ std::vector<core::MusicDataChoice> DirectionWriter::createFiguredBassElements()
             }
         }
 
+        setId(figuredBassData.id, figuredBass);
+
         if (figuredBassData.parentheses != api::Bool::unspecified)
         {
             figuredBass.setParentheses(figuredBassData.parentheses == api::Bool::yes ? core::YesNo::yes()
diff --git a/src/private/mx/impl/IdFunctions.h b/src/private/mx/impl/IdFunctions.h
new file mode 100644
index 000000000..d3cda7b7c
--- /dev/null
+++ b/src/private/mx/impl/IdFunctions.h
@@ -0,0 +1,37 @@
+// MusicXML Class Library
+// Copyright (c) by Matthew James Briggs
+// Distributed under the MIT License
+
+#pragma once
+
+#include "mx/core/Token.h"
+
+#include <optional>
+#include <string>
+
+namespace mx
+{
+namespace impl
+{
+// Read the id attribute from any core element that has one. Absent stays absent.
+template <typename CORE_TYPE> std::optional<std::string> getId(const CORE_TYPE &inCoreElement)
+{
+    if (!inCoreElement.id().has_value())
+    {
+        return std::nullopt;
+    }
+
+    return inCoreElement.id()->value();
+}
+
+// Write the id attribute onto any core element that has one. An absent or empty id writes no
+// attribute. core::Token repairs an id that is not a valid XML name.
+template <typename CORE_TYPE> void setId(const std::optional<std::string> &inId, CORE_TYPE &outCoreElement)
+{
+    if (inId.has_value() && !inId->empty())
+    {
+        outCoreElement.setID(core::Token{*inId});
+    }
+}
+} // namespace impl
+} // namespace mx
diff --git a/src/private/mx/impl/MeasureReader.cpp b/src/private/mx/impl/MeasureReader.cpp
index d917b70a8..8adc66065 100644
--- a/src/private/mx/impl/MeasureReader.cpp
+++ b/src/private/mx/impl/MeasureReader.cpp
@@ -52,6 +52,7 @@
 #include "mx/core/generated/YesNo.h"
 #include "mx/impl/Converter.h"
 #include "mx/impl/DirectionReader.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/NoteFunctions.h"
 #include "mx/impl/NoteReader.h"
 #include "mx/impl/SoundFunctions.h"
@@ -142,6 +143,7 @@ std::pair<api::MeasureData, std::optional<api::TransposeData>> MeasureReader::ge
     std::lock_guard<std::mutex> lock(myMutex);
     myOutMeasureData = api::MeasureData{};
     myOutMeasureData.number = myPartwiseMeasure.number();
+    myOutMeasureData.id = getId(myPartwiseMeasure);
 
     // if we are parsing the first measure of the part, then we need to return transpose information
     std::optional<api::TransposeData> transpose;
@@ -234,10 +236,13 @@ void MeasureReader::parseTimeSignature() const
         staffTimeSignatures = myPreviousMeasureCursor.staffTimeSignatures;
     }
 
+    // A carried-forward time signature is not the <time> element that stated it, so it keeps no id.
     timeSignature.isImplicit = true;
+    timeSignature.id.reset();
     for (auto &entry : staffTimeSignatures)
     {
         entry.second.isImplicit = true;
+        entry.second.id.reset();
     }
 
     TimeReader timeReader{myPartwiseMeasure.musicData()};
@@ -684,6 +689,7 @@ std::optional<api::TransposeData> MeasureReader::parseAttributes(const core::Att
                 keyData.nonTraditional.emplace_back(keyComponent);
             }
 
+            keyData.id = getId(key);
             myOutMeasureData.keys.emplace_back(std::move(keyData));
             continue;
         }
@@ -716,6 +722,7 @@ std::optional<api::TransposeData> MeasureReader::parseAttributes(const core::Att
             keyData.mode = myConverter.convert(*traditionalKey.mode());
         }
         keyData.tickTimePosition = myCurrentCursor.tickTimePosition;
+        keyData.id = getId(key);
 
         myOutMeasureData.keys.emplace_back(std::move(keyData));
     }
@@ -766,6 +773,7 @@ std::optional<api::TransposeData> MeasureReader::parseAttributes(const core::Att
             }
 
             transposeData.tickTimePosition = myCurrentCursor.tickTimePosition;
+            transposeData.id = getId(coreTranspose);
             myOutMeasureData.transpositions.emplace_back(std::move(transposeData));
         }
     }
@@ -829,6 +837,8 @@ void MeasureReader::parseFiguredBass(const core::FiguredBass &inMxFiguredBass, c
         figuredBass.durationTimeTicks = myCurrentCursor.convertDurationToGlobalTickScale(*inMxFiguredBass.duration());
     }
 
+    figuredBass.id = getId(inMxFiguredBass);
+
     auto direction = api::DirectionData{};
     direction.tickTimePosition = myCurrentCursor.tickTimePosition;
     direction.placement = api::Placement::below;
@@ -978,6 +988,7 @@ void MeasureReader::parseBarline(const core::Barline &inMxBarline) const
     barline.repeatDirection = repeatDirection;
     barline.repeatAfterJump = repeatAfterJump;
     barline.repeatWinged = repeatWinged;
+    barline.id = getId(inMxBarline);
     myOutMeasureData.barlines.emplace_back(std::move(barline));
 }
 
@@ -1047,6 +1058,7 @@ void MeasureReader::importClef(const core::Clef &inClef) const
 {
     api::ClefData clefData;
     clefData.tickTimePosition = myCurrentCursor.tickTimePosition;
+    clefData.id = getId(inClef);
     auto converter = Converter{};
     clefData.symbol = converter.convert(inClef.clef().sign());
 
diff --git a/src/private/mx/impl/MeasureWriter.cpp b/src/private/mx/impl/MeasureWriter.cpp
index 4ff2bf2ad..b099084eb 100644
--- a/src/private/mx/impl/MeasureWriter.cpp
+++ b/src/private/mx/impl/MeasureWriter.cpp
@@ -35,6 +35,7 @@
 #include "mx/core/generated/YesNo.h"
 #include "mx/impl/Converter.h"
 #include "mx/impl/DirectionWriter.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/LayoutFunctions.h"
 #include "mx/impl/NoteWriter.h"
 #include "mx/impl/ScoreWriter.h"
@@ -100,6 +101,8 @@ void MeasureWriter::writeMeasureGlobals()
         myOutMeasure.setWidth(core::Tenths{core::Decimal{static_cast<double>(myMeasureData.width)}});
     }
 
+    impl::setId(myMeasureData.id, myOutMeasure);
+
     Converter converter;
 
     if (myMeasureData.implicit != api::Bool::unspecified)
@@ -916,6 +919,7 @@ void MeasureWriter::writeBarlines(int tickTimePosition)
             barlineElement.setRepeat(repeatElement);
         }
 
+        impl::setId(myBarlinesIter->id, barlineElement);
         myOutMeasure.addMusicData(core::MusicDataChoice::barline(barlineElement));
         myHistory.log("writeBarline");
     }
diff --git a/src/private/mx/impl/NotationsWriter.cpp b/src/private/mx/impl/NotationsWriter.cpp
index 6973c3b25..f79da2233 100644
--- a/src/private/mx/impl/NotationsWriter.cpp
+++ b/src/private/mx/impl/NotationsWriter.cpp
@@ -62,6 +62,7 @@
 #include "mx/impl/Converter.h"
 #include "mx/impl/CurveFunctions.h"
 #include "mx/impl/DynamicsWriter.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/MarkDataFunctions.h"
 #include "mx/impl/PositionFunctions.h"
 #include "mx/impl/ScoreWriter.h"
@@ -184,6 +185,7 @@ core::Notations NotationsWriter::getNotations() const
     {
         core::Tuplet tuplet;
         tuplet.setType(core::StartStop::stop());
+        setId(tupletStop.id, tuplet);
 
         if (tupletStop.numberLevel > 0)
         {
@@ -197,6 +199,7 @@ core::Notations NotationsWriter::getNotations() const
     {
         core::Tuplet tuplet;
         tuplet.setType(core::StartStop::start());
+        setId(tupletStart.id, tuplet);
 
         core::TupletPortion actual;
         core::TupletNumber tn1;
diff --git a/src/private/mx/impl/NoteFunctions.cpp b/src/private/mx/impl/NoteFunctions.cpp
index 8ca73c289..705c715df 100644
--- a/src/private/mx/impl/NoteFunctions.cpp
+++ b/src/private/mx/impl/NoteFunctions.cpp
@@ -11,6 +11,7 @@
 #include "mx/impl/CurveFunctions.h"
 #include "mx/impl/DynamicsReader.h"
 #include "mx/impl/FermataFunctions.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/MarkDataFunctions.h"
 #include "mx/impl/NonArpeggiateFunctions.h"
 #include "mx/impl/NoteReader.h"
@@ -159,6 +160,7 @@ api::NoteData NoteFunctions::parseNote() const
 
     parseMiscData();
     myOutNoteData.positionData = impl::getPositionData(myNote);
+    myOutNoteData.id = impl::getId(myNote);
 
     return myOutNoteData;
 }
diff --git a/src/private/mx/impl/NoteReader.cpp b/src/private/mx/impl/NoteReader.cpp
index d95270a18..c714ec141 100644
--- a/src/private/mx/impl/NoteReader.cpp
+++ b/src/private/mx/impl/NoteReader.cpp
@@ -15,6 +15,7 @@
 #include "mx/core/generated/TextElementData.h"
 #include "mx/core/generated/Tied.h"
 #include "mx/impl/FontFunctions.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/PositionFunctions.h"
 #include "mx/impl/PrintFunctions.h"
 #include "mx/utility/StringToInt.h"
@@ -527,6 +528,7 @@ void NoteReader::setLyric()
 
         api::LyricData lyricData;
         lyricData.positionData = getLyricPositionData(lyric);
+        lyricData.id = getId(lyric);
 
         if (lyric.number().has_value())
         {
diff --git a/src/private/mx/impl/NoteWriter.cpp b/src/private/mx/impl/NoteWriter.cpp
index 90f6b18b3..9ed71c384 100644
--- a/src/private/mx/impl/NoteWriter.cpp
+++ b/src/private/mx/impl/NoteWriter.cpp
@@ -27,6 +27,7 @@
 #include "mx/core/generated/TimeModification.h"
 #include "mx/core/generated/TimeModificationGroup.h"
 #include "mx/core/generated/Unpitched.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/NotationsWriter.h"
 #include "mx/impl/PositionFunctions.h"
 #include "mx/impl/PrintFunctions.h"
@@ -85,6 +86,7 @@ core::Note NoteWriter::getNote(bool isStartOfChord) const
     setNotehead();
     setStemDirection();
     setMiscData();
+    impl::setId(myNoteData.id, myOutNote);
     NotationsWriter notationsWriter{myNoteData, myCursor, myScoreWriter};
     impl::setAttributesFromPositionData(myNoteData.positionData, myOutNote);
     if (myNoteData.printData.printObject != api::Bool::unspecified)
@@ -559,6 +561,7 @@ void NoteWriter::setLyrics() const
         }
 
         impl::setAttributesFromPositionData(lyricData.positionData, lyric);
+        impl::setId(lyricData.id, lyric);
         if (lyricData.positionData.horizontalAlignment != api::HorizontalAlignment::unspecified)
         {
             lyric.setJustify(myConverter.convert(lyricData.positionData.horizontalAlignment));
diff --git a/src/private/mx/impl/PropertiesWriter.cpp b/src/private/mx/impl/PropertiesWriter.cpp
index b80a5faf9..99bbe0a0d 100644
--- a/src/private/mx/impl/PropertiesWriter.cpp
+++ b/src/private/mx/impl/PropertiesWriter.cpp
@@ -40,6 +40,7 @@
 #include "mx/core/generated/Transpose.h"
 #include "mx/core/generated/YesNo.h"
 #include "mx/impl/Converter.h"
+#include "mx/impl/IdFunctions.h"
 
 namespace mx
 {
@@ -94,6 +95,7 @@ void PropertiesWriter::writeMultipleRest(int measureCount, api::Bool useSymbols)
 void PropertiesWriter::writeKey(int staffIndex, const api::KeyData &inKeyData)
 {
     core::Key key{};
+    impl::setId(inKeyData.id, key);
 
     if (staffIndex >= 0)
     {
@@ -234,6 +236,7 @@ void PropertiesWriter::writeTime(const api::TimeChoice &value, int staffIndex)
 {
     Converter converter;
     core::Time time{};
+    impl::setId(value.id, time);
 
     if (value.isSimple())
     {
@@ -325,6 +328,7 @@ void PropertiesWriter::writeStaffDetails(int staffIndex, int staffLines, double
 void PropertiesWriter::writeClef(int staffIndex, const api::ClefData &inClefData)
 {
     core::Clef mxClef{};
+    impl::setId(inClefData.id, mxClef);
 
     // staffIndex < 0 means a single-staff part (the caller's clefStaffIndex() collapses it), so the
     // auto rule omits the implied 1; staffIndex >= 0 is a multi-staff part, so the auto rule emits
@@ -403,6 +407,7 @@ void PropertiesWriter::writePartSymbol(const api::PartSymbolData &inPartSymbolDa
 void PropertiesWriter::writeTranspose(int staffIndex, const api::TransposeData &inTransposeData)
 {
     auto xpose = Converter::convertToTranspose(inTransposeData);
+    impl::setId(inTransposeData.id, xpose);
 
     if (staffIndex >= 0)
     {
diff --git a/src/private/mx/impl/SoundFunctions.cpp b/src/private/mx/impl/SoundFunctions.cpp
index b1f0c1995..a9f94f851 100644
--- a/src/private/mx/impl/SoundFunctions.cpp
+++ b/src/private/mx/impl/SoundFunctions.cpp
@@ -12,6 +12,7 @@
 #include "mx/core/generated/SwingChoiceGroup.h"
 #include "mx/core/generated/SwingTypeValue.h"
 #include "mx/core/generated/YesNo.h"
+#include "mx/impl/IdFunctions.h"
 
 namespace mx
 {
@@ -144,11 +145,15 @@ api::SoundData readSoundData(const core::Sound &inSound)
         out.swing = soundFunctionsToApiSwingData(*inSound.swing());
     }
 
+    out.id = getId(inSound);
+
     return out;
 }
 
 void writeSoundData(const api::SoundData &inSoundData, core::Sound &outSound)
 {
+    setId(inSoundData.id, outSound);
+
     if (inSoundData.tempo >= 0.0)
     {
         outSound.setTempo(core::NonNegativeDecimal{core::Decimal{inSoundData.tempo}});
diff --git a/src/private/mx/impl/SpannerFunctions.h b/src/private/mx/impl/SpannerFunctions.h
index 4116cd199..1baa8cc4a 100644
--- a/src/private/mx/impl/SpannerFunctions.h
+++ b/src/private/mx/impl/SpannerFunctions.h
@@ -5,10 +5,10 @@
 #pragma once
 
 #include "mx/api/SpannerData.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/LineFunctions.h"
 #include "mx/impl/PositionFunctions.h"
 #include "mx/impl/PrintFunctions.h"
-#include "mx/utility/Unused.h"
 
 #include <optional>
 
@@ -29,6 +29,7 @@ template <typename ATTRIBUTES_TYPE> api::SpannerStart getSpannerStart(const ATTR
     start.positionData = getPositionData(inAttributes);
     start.printData = getPrintData(inAttributes);
     start.lineData = getLineData(inAttributes);
+    start.id = getId(inAttributes);
     return start;
 }
 
@@ -41,6 +42,7 @@ template <typename ATTRIBUTES_TYPE> api::SpannerStop getSpannerStop(const ATTRIB
     }
     stop.positionData = getPositionData(inAttributes);
     stop.lineData = getLineData(inAttributes);
+    stop.id = getId(inAttributes);
     return stop;
 }
 
@@ -53,7 +55,8 @@ template <typename ATTRIBUTES_TYPE>
 void setAttributesFromSpannerStart(const api::SpannerStart &start, ATTRIBUTES_TYPE &outAttributes,
                                    const std::optional<int> &inResolvedNumber)
 {
-    MX_UNUSED(start);
+    setId(start.id, outAttributes);
+
     if (inResolvedNumber.has_value())
     {
         lookForAndSetHasNumber(true, &outAttributes);
@@ -81,6 +84,7 @@ void setAttributesFromSpannerStop(const api::SpannerStop &stop, ATTRIBUTES_TYPE
 
     setAttributesFromPositionData(stop.positionData, outAttributes);
     setAttributesFromLineData(stop.lineData, outAttributes);
+    setId(stop.id, outAttributes);
 }
 } // namespace impl
 } // namespace mx
diff --git a/src/private/mx/impl/TimeReader.cpp b/src/private/mx/impl/TimeReader.cpp
index b412bfd71..f94bbd577 100644
--- a/src/private/mx/impl/TimeReader.cpp
+++ b/src/private/mx/impl/TimeReader.cpp
@@ -11,6 +11,7 @@
 #include "mx/core/generated/TimeChoiceGroup.h"
 #include "mx/core/generated/TimeSignatureGroup.h"
 #include "mx/impl/Converter.h"
+#include "mx/impl/IdFunctions.h"
 
 #include <utility>
 #include <vector>
@@ -100,6 +101,7 @@ TimeReaderResult TimeReader::createTimeChoice(const core::Time &inTime)
     }
 
     timeChoice.isImplicit = false;
+    timeChoice.id = getId(inTime);
     if (inTime.printObject().has_value())
     {
         timeChoice.display = converter.convert(*inTime.printObject());
diff --git a/src/private/mx/impl/TupletReader.cpp b/src/private/mx/impl/TupletReader.cpp
index 12605fdf6..900a94457 100644
--- a/src/private/mx/impl/TupletReader.cpp
+++ b/src/private/mx/impl/TupletReader.cpp
@@ -9,6 +9,7 @@
 #include "mx/core/generated/Tuplet.h"
 #include "mx/core/generated/TupletPortion.h"
 #include "mx/impl/Converter.h"
+#include "mx/impl/IdFunctions.h"
 #include "mx/impl/MarkDataFunctions.h"
 
 namespace mx
@@ -25,6 +26,7 @@ void TupletReader::parseTuplet(std::vector<api::TupletStart> &outTupletStarts,
 {
     api::TupletStart tupletStart;
     tupletStart.positionData = impl::getPositionData(myTuplet);
+    tupletStart.id = getId(myTuplet);
 
     if (myTuplet.number().has_value())
     {
@@ -36,6 +38,7 @@ void TupletReader::parseTuplet(std::vector<api::TupletStart> &outTupletStarts,
         api::TupletStop tupletStop;
         tupletStop.positionData = tupletStart.positionData;
         tupletStop.numberLevel = tupletStart.numberLevel;
+        tupletStop.id = getId(myTuplet);
         outTupletStops.emplace_back(std::move(tupletStop));
         return;
     }
diff --git a/src/private/mxtest/api/IdAttributeApiTest.cpp b/src/private/mxtest/api/IdAttributeApiTest.cpp
new file mode 100644
index 000000000..c9d3ed7eb
--- /dev/null
+++ b/src/private/mxtest/api/IdAttributeApiTest.cpp
@@ -0,0 +1,429 @@
+// MusicXML Class Library
+// Copyright (c) by Matthew James Briggs
+// Distributed under the MIT License
+
+#include "mxtest/control/CompileControl.h"
+#ifdef MX_COMPILE_API_TESTS
+
+#include "cpul/cpulTestHarness.h"
+#include "mx/api/DocumentManager.h"
+#include "mx/api/ScoreData.h"
+#include "mxtest/api/RoundTrip.h"
+#include "mxtest/api/TestHelpers.h"
+
+using namespace std;
+using namespace mx::api;
+using namespace mxtest;
+
+// A one-part, one-measure score holding a single quarter note, ready for a test to hang ids on.
+static ScoreData idAttributeMakeScore()
+{
+    ScoreData score;
+    score.ticksPerQuarter = 4;
+    score.parts.emplace_back();
+    auto &part = score.parts.back();
+    part.name = "P1";
+    part.uniqueId = "P1";
+    part.measures.emplace_back();
+    auto &measure = part.measures.back();
+    measure.staves.emplace_back();
+    auto &staff = measure.staves.back();
+
+    NoteData note{};
+    note.pitchData.step = Step::c;
+    note.pitchData.octave = 4;
+    note.durationData.durationTimeTicks = 4;
+    note.durationData.durationName = DurationName::quarter;
+    note.tickTimePosition = 0;
+    staff.voices[0].notes.push_back(note);
+
+    return score;
+}
+
+static MeasureData &idAttributeMeasure(ScoreData &score)
+{
+    return score.parts.back().measures.back();
+}
+
+static const MeasureData &idAttributeMeasure(const ScoreData &score)
+{
+    return score.parts.back().measures.back();
+}
+
+static NoteData &idAttributeNote(ScoreData &score)
+{
+    return idAttributeMeasure(score).staves.back().voices.at(0).notes.back();
+}
+
+static const NoteData &idAttributeNote(const ScoreData &score)
+{
+    return idAttributeMeasure(score).staves.back().voices.at(0).notes.back();
+}
+
+// The <note> id, the reason for issue 399, plus the <measure> and <lyric> ids around it.
+TEST(noteMeasureAndLyricIds, IdAttribute)
+{
+    auto score = idAttributeMakeScore();
+    idAttributeMeasure(score).id = "m1";
+
+    auto &note = idAttributeNote(score);
+    note.id = "n1";
+
+    LyricData lyric;
+    lyric.text = "la";
+    lyric.id = "ly1";
+    note.lyrics.push_back(lyric);
+
+    const auto xml = toXml(score);
+    CHECK(xml.find("id=\"n1\"") != std::string::npos);
+
+    const auto out = roundTrip(score);
+    CHECK(std::optional<std::string>{"m1"} == idAttributeMeasure(out).id);
+
+    const auto &onote = idAttributeNote(out);
+    CHECK(std::optional<std::string>{"n1"} == onote.id);
+    REQUIRE(onote.lyrics.size() == 1);
+    CHECK(std::optional<std::string>{"ly1"} == onote.lyrics.front().id);
+}
+
+T_END;
+
+// The ids on the elements that live in <attributes>, plus the <barline> id.
+TEST(attributesAndBarlineIds, IdAttribute)
+{
+    auto score = idAttributeMakeScore();
+    auto &measure = idAttributeMeasure(score);
+
+    measure.timeSignature = TimeChoice{TimeSignatureData{}};
+    measure.timeSignature.isImplicit = false;
+    measure.timeSignature.id = "t1";
+
+    KeyData key;
+    key.fifths = 2;
+    key.mode = KeyMode::major;
+    key.id = "k1";
+    measure.keys.push_back(key);
+
+    ClefData clef;
+    clef.setTreble();
+    clef.id = "c1";
+    measure.staves.back().clefs.push_back(clef);
+
+    TransposeData transpose{-2, -1};
+    transpose.id = "x1";
+    measure.transpositions.push_back(transpose);
+
+    BarlineData barline;
+    barline.barlineType = BarlineType::lightHeavy;
+    barline.location = HorizontalAlignment::right;
+    barline.id = "b1";
+    measure.barlines.push_back(barline);
+
+    const auto out = roundTrip(score);
+    const auto &omeasure = idAttributeMeasure(out);
+
+    CHECK(std::optional<std::string>{"t1"} == omeasure.timeSignature.id);
+    REQUIRE(omeasure.keys.size() == 1);
+    CHECK(std::optional<std::string>{"k1"} == omeasure.keys.front().id);
+    REQUIRE(omeasure.staves.back().clefs.size() == 1);
+    CHECK(std::optional<std::string>{"c1"} == omeasure.staves.back().clefs.front().id);
+    REQUIRE(omeasure.transpositions.size() == 1);
+    CHECK(std::optional<std::string>{"x1"} == omeasure.transpositions.front().id);
+    REQUIRE(omeasure.barlines.size() == 1);
+    CHECK(std::optional<std::string>{"b1"} == omeasure.barlines.front().id);
+}
+
+T_END;
+
+// A time signature that a later measure inherits carries no id: only the measure that states the
+// <time> element owns it.
+TEST(inheritedTimeSignatureHasNoId, IdAttribute)
+{
+    auto score = idAttributeMakeScore();
+    auto &measure = idAttributeMeasure(score);
+    measure.timeSignature = TimeChoice{TimeSignatureData{}};
+    measure.timeSignature.isImplicit = false;
+    measure.timeSignature.id = "t1";
+
+    auto &part = score.parts.back();
+    part.measures.push_back(part.measures.back());
+    part.measures.back().timeSignature.isImplicit = true;
+    part.measures.back().id = std::nullopt;
+
+    const auto out = roundTrip(score);
+    REQUIRE(out.parts.back().measures.size() == 2);
+    CHECK(std::optional<std::string>{"t1"} == out.parts.back().measures.front().timeSignature.id);
+    CHECK(!out.parts.back().measures.back().timeSignature.id.has_value());
+}
+
+T_END;
+
+// The <direction> id and the ids of the direction-type elements mx::api models as their own types.
+TEST(directionIds, IdAttribute)
+{
+    auto score = idAttributeMakeScore();
+    auto &staff = idAttributeMeasure(score).staves.back();
+
+    DirectionData direction;
+    direction.tickTimePosition = 0;
+    direction.id = "d1";
+
+    WordsData words;
+    words.text = "dolce";
+    words.id = "w1";
+
+    SymbolData symbol;
+    symbol.smufl = "arrowBlackUp";
+    symbol.id = "sy1";
+
+    direction.directionTypes.emplace_back(
+        DirectionChoice{std::vector<WordsChoice>{WordsChoice{words}, WordsChoice{symbol}}});
+
+    RehearsalData rehearsal;
+    rehearsal.text = "A";
+    rehearsal.id = "r1";
+    direction.directionTypes.emplace_back(DirectionChoice{rehearsal});
+
+    WedgeStart wedgeStart;
+    wedgeStart.wedgeType = WedgeType::crescendo;
+    wedgeStart.id = "we1";
+    direction.directionTypes.emplace_back(DirectionChoice{wedgeStart});
+
+    SpannerStart bracketStart;
+    bracketStart.id = "br1";
+    direction.directionTypes.emplace_back(DirectionChoice::bracketStart(bracketStart));
+
+    SpannerStart dashesStart;
+    dashesStart.id = "da1";
+    direction.directionTypes.emplace_back(DirectionChoice::dashesStart(dashesStart));
+
+    OttavaStart ottavaStart;
+    ottavaStart.ottavaType = OttavaType::o8va;
+    ottavaStart.spannerStart.id = "o1";
+    direction.directionTypes.emplace_back(DirectionChoice{ottavaStart});
+
+    PedalLineData pedal{PedalLineKind::start};
+    pedal.id = "p1";
+    direction.directionTypes.emplace_back(DirectionChoice{pedal});
+
+    direction.isSoundDataSpecified = true;
+    direction.soundData.tempo = 120.0;
+    direction.soundData.id = "so1";
+
+    staff.directions.push_back(direction);
+
+    const auto out = roundTrip(score);
+    const auto &odirections = idAttributeMeasure(out).staves.back().directions;
+    REQUIRE(odirections.size() == 1);
+    const auto &odirection = odirections.front();
+
+    CHECK(std::optional<std::string>{"d1"} == odirection.id);
+    CHECK(std::optional<std::string>{"so1"} == odirection.soundData.id);
+
+    std::optional<std::string> foundWords;
+    std::optional<std::string> foundSymbol;
+    std::optional<std::string> foundRehearsal;
+    std::optional<std::string> foundWedge;
+    std::optional<std::string> foundBracket;
+    std::optional<std::string> foundDashes;
+    std::optional<std::string> foundOttava;
+    std::optional<std::string> foundPedal;
+
+    for (const auto &choice : odirection.directionTypes)
+    {
+        switch (choice.kind())
+        {
+        case DirectionChoice::Kind::wordsRun:
+            for (const auto &item : choice.wordsRun())
+            {
+                if (item.isWords())
+                {
+                    foundWords = item.words().id;
+                }
+                else if (item.isSymbol())
+                {
+                    foundSymbol = item.symbol().id;
+                }
+            }
+            break;
+        case DirectionChoice::Kind::rehearsal:
+            foundRehearsal = choice.rehearsal().id;
+            break;
+        case DirectionChoice::Kind::wedgeStart:
+            foundWedge = choice.wedgeStart().id;
+            break;
+        case DirectionChoice::Kind::bracketStart:
+            foundBracket = choice.bracketStart().id;
+            break;
+        case DirectionChoice::Kind::dashesStart:
+            foundDashes = choice.dashesStart().id;
+            break;
+        case DirectionChoice::Kind::ottavaStart:
+            foundOttava = choice.ottavaStart().spannerStart.id;
+            break;
+        case DirectionChoice::Kind::pedal:
+            foundPedal = choice.pedal().id;
+            break;
+        default:
+            break;
+        }
+    }
+
+    CHECK(std::optional<std::string>{"w1"} == foundWords);
+    CHECK(std::optional<std::string>{"sy1"} == foundSymbol);
+    CHECK(std::optional<std::string>{"r1"} == foundRehearsal);
+    CHECK(std::optional<std::string>{"we1"} == foundWedge);
+    CHECK(std::optional<std::string>{"br1"} == foundBracket);
+    CHECK(std::optional<std::string>{"da1"} == foundDashes);
+    CHECK(std::optional<std::string>{"o1"} == foundOttava);
+    CHECK(std::optional<std::string>{"p1"} == foundPedal);
+}
+
+T_END;
+
+// The ids on the note-attached elements: <slur>, <tuplet> and a let-ring <tied>.
+TEST(notationIds, IdAttribute)
+{
+    auto score = idAttributeMakeScore();
+    auto &staff = idAttributeMeasure(score).staves.back();
+    auto &notes = staff.voices.at(0).notes;
+
+    NoteData second = notes.back();
+    second.tickTimePosition = 4;
+    notes.push_back(second);
+
+    CurveStart slurStart{CurveType::slur};
+    slurStart.id = "s1";
+    notes.front().noteAttachmentData.curveStarts.push_back(slurStart);
+
+    CurveStop slurStop{CurveType::slur};
+    slurStop.id = "s2";
+    notes.back().noteAttachmentData.curveStops.push_back(slurStop);
+
+    TupletStart tupletStart;
+    tupletStart.actualNumber = 3;
+    tupletStart.actualDurationName = DurationName::quarter;
+    tupletStart.normalNumber = 2;
+    tupletStart.normalDurationName = DurationName::quarter;
+    tupletStart.id = "tu1";
+    notes.front().noteAttachmentData.tupletStarts.push_back(tupletStart);
+
+    TupletStop tupletStop;
+    tupletStop.id = "tu2";
+    notes.back().noteAttachmentData.tupletStops.push_back(tupletStop);
+
+    TieLetRing letRing;
+    letRing.id = "lr1";
+    notes.back().tieLetRing = letRing;
+
+    const auto out = roundTrip(score);
+    const auto &onotes = idAttributeMeasure(out).staves.back().voices.at(0).notes;
+    REQUIRE(onotes.size() == 2);
+
+    REQUIRE(onotes.front().noteAttachmentData.curveStarts.size() == 1);
+    CHECK(std::optional<std::string>{"s1"} == onotes.front().noteAttachmentData.curveStarts.front().id);
+    REQUIRE(onotes.back().noteAttachmentData.curveStops.size() == 1);
+    CHECK(std::optional<std::string>{"s2"} == onotes.back().noteAttachmentData.curveStops.front().id);
+
+    REQUIRE(onotes.front().noteAttachmentData.tupletStarts.size() == 1);
+    CHECK(std::optional<std::string>{"tu1"} == onotes.front().noteAttachmentData.tupletStarts.front().id);
+    REQUIRE(onotes.back().noteAttachmentData.tupletStops.size() == 1);
+    CHECK(std::optional<std::string>{"tu2"} == onotes.back().noteAttachmentData.tupletStops.front().id);
+
+    REQUIRE(onotes.back().tieLetRing.has_value());
+    CHECK(std::optional<std::string>{"lr1"} == onotes.back().tieLetRing->id);
+}
+
+T_END;
+
+// The <figured-bass> id.
+TEST(figuredBassId, IdAttribute)
+{
+    auto score = idAttributeMakeScore();
+    auto &staff = idAttributeMeasure(score).staves.back();
+
+    FigureData figure;
+    figure.figureNumber = "6";
+
+    FiguredBassData figuredBass;
+    figuredBass.figures.push_back(figure);
+    figuredBass.id = "fb1";
+
+    DirectionData direction;
+    direction.tickTimePosition = 0;
+    direction.placement = Placement::below;
+    direction.figuredBasses.push_back(figuredBass);
+    staff.directions.push_back(direction);
+
+    const auto out = roundTrip(score);
+    const auto &odirections = idAttributeMeasure(out).staves.back().directions;
+    REQUIRE(odirections.size() == 1);
+    REQUIRE(odirections.front().figuredBasses.size() == 1);
+    CHECK(std::optional<std::string>{"fb1"} == odirections.front().figuredBasses.front().id);
+}
+
+T_END;
+
+// Reading ids straight off a MusicXML source.
+TEST(idsAreReadFromSourceXml, IdAttribute)
+{
+    const std::string xml = R"(<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE score-partwise PUBLIC "-//Recordare//DTD MusicXML 4.0 Partwise//EN" "http://www.musicxml.org/dtds/partwise.dtd">
+<score-partwise version="4.0">
+  <part-list>
+    <score-part id="P1">
+      <part-name>Music</part-name>
+    </score-part>
+  </part-list>
+  <part id="P1">
+    <measure number="1" id="m1">
+      <attributes>
+        <divisions>1</divisions>
+        <key id="k1">
+          <fifths>0</fifths>
+        </key>
+        <time id="t1">
+          <beats>4</beats>
+          <beat-type>4</beat-type>
+        </time>
+        <clef id="c1">
+          <sign>G</sign>
+          <line>2</line>
+        </clef>
+      </attributes>
+      <note id="n1">
+        <pitch>
+          <step>C</step>
+          <octave>4</octave>
+        </pitch>
+        <duration>4</duration>
+        <type>whole</type>
+      </note>
+      <barline location="right" id="b1">
+        <bar-style>light-heavy</bar-style>
+      </barline>
+    </measure>
+  </part>
+</score-partwise>)";
+
+    const auto score = fromXml(xml);
+    REQUIRE(score.parts.size() == 1);
+    const auto &measure = score.parts.front().measures.front();
+
+    CHECK(std::optional<std::string>{"m1"} == measure.id);
+    CHECK(std::optional<std::string>{"t1"} == measure.timeSignature.id);
+    REQUIRE(measure.keys.size() == 1);
+    CHECK(std::optional<std::string>{"k1"} == measure.keys.front().id);
+    REQUIRE(measure.staves.front().clefs.size() == 1);
+    CHECK(std::optional<std::string>{"c1"} == measure.staves.front().clefs.front().id);
+    REQUIRE(measure.barlines.size() == 1);
+    CHECK(std::optional<std::string>{"b1"} == measure.barlines.front().id);
+
+    const auto &notes = measure.staves.front().voices.at(0).notes;
+    REQUIRE(notes.size() == 1);
+    CHECK(std::optional<std::string>{"n1"} == notes.front().id);
+}
+
+T_END;
+
+#endif
diff --git a/src/private/mxtest/api/roundtrip-baseline.txt b/src/private/mxtest/api/roundtrip-baseline.txt
index 72a311cac..26ce9b255 100644
--- a/src/private/mxtest/api/roundtrip-baseline.txt
+++ b/src/private/mxtest/api/roundtrip-baseline.txt
@@ -639,3 +639,8 @@ synthetic/harmonic.3.0.xml
 # attribute: this file's number="1, 2, 3" came back as number="1". BarlineData::ending
 # is now an optional EndingData holding the whole number list plus the display text.
 lysuite/ly45f_Repeats_InvalidEndings.xml
+
+# Unblocked by #399 (MusicXML id attributes). The id attribute of <bracket> and
+# <dashes> reaches the api on SpannerStart/SpannerStop, so it is no longer dropped.
+synthetic/bracket.3.1.xml
+synthetic/dashes.3.1.xml