diff --git a/tools/drift-check b/tools/drift-check
index b3bddf0..939ca8a 100755
--- a/tools/drift-check
+++ b/tools/drift-check
@@ -25,6 +25,13 @@ same lecture. This script compares all of that against the reconstructed
   that is merely stale (canonical moved, the copy did not), an ``interim``
   entry whose divergent copy moved again, or a divergence that converged.
 
+A non-canonical copy is judged by the rule promotion admitted it under: it is
+the same lecture as the canonical copy when it is byte-identical, or identical
+once each copy's asset references are normalised to their pool paths (series
+spell ``/_static/...`` and ``_static/...`` for the same file). That comparison
+is ``tools/promote``'s own ``normalised_text``, imported for the purpose, so
+the two tools cannot disagree about which copies are one lecture.
+
 Two kinds of copy are deliberately NOT watched. A consumer series' copies
 (``class: consumer`` in ``sync/manifest.yml``, recorded under ``superseded:``
 in the ledger, or under ``sources``/``divergent`` by a ledger written before
@@ -169,6 +176,45 @@ def consumer_series(manifest: dict) -> set:
     return {k for k, v in manifest.items() if (v or {}).get("class") == CONSUMER_CLASS}
 
 
+PROMOTE_PATH = Path(__file__).resolve().parent / "promote"
+_promote = None
+
+
+def promote_module():
+    """``tools/promote`` imported as a module, once, on first use. Only its
+    ``normalised_text`` is wanted: the form two same-name copies are compared
+    in at promotion. Importing it (rather than copying the asset parser here)
+    keeps the drift-check's idea of "one lecture" identical to promote's."""
+    global _promote
+    if _promote is None:
+        import importlib.util
+        from importlib.machinery import SourceFileLoader
+
+        loader = SourceFileLoader("qe_promote", str(PROMOTE_PATH))
+        spec = importlib.util.spec_from_loader(loader.name, loader)
+        module = importlib.util.module_from_spec(spec)
+        loader.exec_module(module)
+        _promote = module
+    return _promote
+
+
+def same_lecture(series: str, canonical: str, path: str, cur: bytes, can_bytes: bytes) -> bool:
+    """Is the ``series`` copy at ``path`` the same lecture as ``canonical``'s,
+    by promotion's rule: byte-identical, or identical once each copy's asset
+    references are normalised to the pool paths promotion would give them.
+    A copy that is not UTF-8 text is never the same lecture."""
+    if cur == can_bytes:
+        return True
+    if not path.endswith(".md"):
+        return False
+    slug = path[:-3]
+    promote = promote_module()
+    try:
+        return promote.normalised_text(series, slug) == promote.normalised_text(canonical, slug)
+    except UnicodeDecodeError:
+        return False
+
+
 # --------------------------------------------------------------------------- #
 # Findings                                                                     #
 # --------------------------------------------------------------------------- #
@@ -348,20 +394,23 @@ def check_lecture(key: str, entry: dict, manifest: dict, consumers: set, finding
                     data={"series": series, "path": s["path"]},
                 ))
             continue
-        if cur == can_bytes:
+        if same_lecture(series, canonical, s["path"], cur, can_bytes):
+            exact = cur == can_bytes
             findings.append(Finding(
                 "info", "converged", key,
-                f"{key}: the {series} copy changed and now matches {canonical} exactly",
-                data={"series": series, "path": s["path"]},
+                f"{key}: the {series} copy changed and now matches {canonical}"
+                + (" exactly" if exact else " up to asset path spelling"),
+                data={"series": series, "path": s["path"], "exact": exact},
             ))
             continue
         findings.append(Finding(
             "drift", "diverged", key,
             f"{key}: the {series} copy lectures/{s['path']} no longer matches its canonical home in {canonical}",
             detail=(
-                f"At promotion the {series} copy was byte-identical to {canonical}'s. It has since been edited "
-                f"on its own. Under the upstream-first rule the fix belongs in {canonical} (and flows down); "
-                f"if the edit is a deliberate variant, record it as one rather than leaving a silent fork."
+                f"At promotion the {series} copy was the same lecture as {canonical}'s (byte-identical, or "
+                f"identical up to asset path spelling). It has since been edited on its own. Under the "
+                f"upstream-first rule the fix belongs in {canonical} (and flows down); if the edit is a "
+                f"deliberate variant, record it as one rather than leaving a silent fork."
             ),
             diff=unified_diff(can_bytes, cur, f"{canonical}/lectures/{can_src['path']}", f"{series}/lectures/{s['path']}"),
             issue_title=f"drift({series}): lectures/{s['path']} differs from its canonical copy in {canonical}",
@@ -384,11 +433,14 @@ def check_lecture(key: str, entry: dict, manifest: dict, consumers: set, finding
             continue
         if sha256_bytes(cur) == d.get("digest"):
             continue
-        if cur == can_bytes:
+        if same_lecture(series, canonical, d["path"], cur, can_bytes):
+            exact = cur == can_bytes
             findings.append(Finding(
                 "info", "converged", key,
-                f"{key}: the divergent {series} copy now matches {canonical} exactly — the interim entry can be re-promoted",
-                data={"series": series, "path": d["path"], "interim": interim},
+                f"{key}: the divergent {series} copy now matches {canonical}"
+                + (" exactly" if exact else " up to asset path spelling")
+                + " — the interim entry can be re-promoted",
+                data={"series": series, "path": d["path"], "interim": interim, "exact": exact},
             ))
         else:
             findings.append(Finding(
diff --git a/tools/tests/test_drift_check_normalised.py b/tools/tests/test_drift_check_normalised.py
new file mode 100644
index 0000000..11f15b4
--- /dev/null
+++ b/tools/tests/test_drift_check_normalised.py
@@ -0,0 +1,79 @@
+"""drift-check judges a non-canonical copy by promotion's own rule: the same
+lecture when byte-identical, or identical once asset references are
+normalised to their pool paths (QuantEcon/lectures#15)."""
+
+from conftest import lecture_text
+
+FIG = "_static/lecture_specific/short_path/graph.png"
+
+
+def short_path_repo(make_repo):
+    """intro (canonical) spells the figure root-relative, intermediate
+    file-relative; promote admits both as one lecture."""
+    repo = make_repo(["intro", "intermediate"])
+    for series in ("intro", "intermediate"):
+        repo.mirror_file(series, FIG, b"png")
+    repo.lecture("intro", "short_path", lecture_text("Shortest Paths", refs=["/" + FIG]))
+    repo.lecture("intermediate", "short_path", lecture_text("Shortest Paths", refs=[FIG]))
+    res = repo.promote("short_path")
+    assert res.returncode == 0, res.stderr
+    entry = repo.entry("short_path")
+    assert [s["series"] for s in entry["sources"]] == ["intro", "intermediate"]
+    assert entry["sources"][0]["digest"] != entry["sources"][1]["digest"]
+    assert repo.drift_check().returncode == 0
+    return repo
+
+
+def test_a_source_moving_in_lockstep_with_canonical_is_not_drift(make_repo):
+    repo = short_path_repo(make_repo)
+
+    # The same upstream edit lands in both series; the spelling still differs.
+    repo.lecture("intro", "short_path", lecture_text("Shortest Paths", "edited", refs=["/" + FIG]))
+    repo.lecture("intermediate", "short_path", lecture_text("Shortest Paths", "edited", refs=[FIG]))
+    res = repo.drift_check()
+
+    assert res.returncode == 1, res.stdout  # the refresh finding alone
+    assert "[canonical-moved]" in res.stdout
+    assert "[diverged]" not in res.stdout
+    assert "[converged] lectures/short_path.md: the intermediate copy changed and now matches intro up to asset path spelling" in res.stdout
+
+
+def test_a_source_changing_only_its_asset_spelling_is_not_drift(make_repo):
+    repo = short_path_repo(make_repo)
+
+    # intermediate adopts a third spelling of the same reference.
+    repo.lecture("intermediate", "short_path", lecture_text("Shortest Paths", refs=["./" + FIG]))
+    res = repo.drift_check()
+
+    assert res.returncode == 0, res.stdout
+    assert "[diverged]" not in res.stdout
+    assert "up to asset path spelling" in res.stdout
+
+
+def test_a_source_edited_on_its_own_is_still_drift(make_repo):
+    repo = short_path_repo(make_repo)
+
+    repo.lecture("intermediate", "short_path", lecture_text("Shortest Paths", "edited in intermediate", refs=[FIG]))
+    res = repo.drift_check()
+
+    assert res.returncode == 1
+    assert "[diverged] lectures/short_path.md: the intermediate copy lectures/short_path.md no longer matches its canonical home in intro" in res.stdout
+
+
+def test_a_divergent_copy_converging_up_to_spelling_is_reported_as_converged(make_repo):
+    repo = make_repo(["intro", "intermediate"])
+    for series in ("intro", "intermediate"):
+        repo.mirror_file(series, FIG, b"png")
+    repo.lecture("intro", "short_path", lecture_text("Shortest Paths", refs=["/" + FIG]))
+    repo.lecture("intermediate", "short_path", lecture_text("Shortest Paths (intermediate)", refs=[FIG]))
+    repo.write_canonical_map({"short_path": "intro"})
+    assert repo.promote("short_path").returncode == 0
+    assert repo.entry("short_path")["interim"] is True
+    assert repo.drift_check().returncode == 0
+
+    # intermediate converges on intro's text, keeping its own spelling.
+    repo.lecture("intermediate", "short_path", lecture_text("Shortest Paths", refs=[FIG]))
+    res = repo.drift_check()
+
+    assert res.returncode == 0, res.stdout
+    assert "[converged] lectures/short_path.md: the divergent intermediate copy now matches intro up to asset path spelling — the interim entry can be re-promoted" in res.stdout