diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index ed6038d..5006692 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -119,3 +119,20 @@ We follow [Conventional Commits](https://www.conventionalcommits.org/):
 [optional body]
 
 [optional footer]
+
+## Signed commits
+
+Every commit that reaches the default branch must be signed; a ruleset refuses
+unsigned pushes. Estate policy:
+[SIGNING-POLICY](https://github.com/hyperpolymath/standards/blob/main/docs/SIGNING-POLICY.adoc).
+
+- **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 is enabled in the tracked repository settings. The repository still requires signed commits on the default branch.
diff --git a/CONTRIBUTING.adoc b/CONTRIBUTING.adoc
index 5949bf0..bb1a67d 100644
--- a/CONTRIBUTING.adoc
+++ b/CONTRIBUTING.adoc
@@ -7,3 +7,23 @@
 . Submit a pull request
 
 *Author:* Jonathan D.A. Jewell j.d.a.jewell@open.ac.uk
+
+== 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. Rewrite the PR
+  branch with signed commits (`git cherry-pick -S`), or squash and sign it
+  locally, then push the updated branch. Create a new PR only when the existing
+  PR branch cannot be rewritten.
+  Rebase-merge is enabled in the tracked repository settings. The repository
+  still requires signed commits on the default branch.