diff --git a/CONTRIBUTING.adoc b/CONTRIBUTING.adoc
index c160882..1cac254 100644
--- a/CONTRIBUTING.adoc
+++ b/CONTRIBUTING.adoc
@@ -130,3 +130,20 @@ licence — see link:LICENSE[LICENSE]. Code is *MPL-2.0*; documentation is
 
 [.text-center]
 *Thank you for contributing!* Remember the motto: _Learn — to love it!_ 💙
+
+== Signed commits
+
+Every commit that reaches the default branch must be signed; a ruleset refuses
+unsigned pushes. Estate policy:
+https://github.com/hyperpolymath/standards/blob/main/docs/SIGNING-POLICY.adoc[SIGNING-POLICY].
+
+* **People and interactive agents** sign with an SSH key registered on GitHub
+  as a *signing* key (`gpg.format=ssh`, `user.signingkey=<key>.pub`,
+  `commit.gpgsign=true`). The committer email must be verified on that account.
+* **Apps, bots and workflows** never `git push` local commits. They write
+  through the API (`createCommitOnBranch` or the estate `signed-push` action)
+  so that GitHub signs each commit.
+* Merge PRs with **squash**. The ruleset checks every commit on the PR branch,
+  not just the result, so one unsigned commit blocks the merge. Re-create such a
+  branch with signed commits (`git cherry-pick -S`) and open a new PR.
+  Rebase-merge replays commits unsigned and is disabled.