diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index dd2a929..dd9c517 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -92,3 +92,21 @@ Conduct](CODE_OF_CONDUCT.md).
 
 By contributing, you agree that your contributions will be licensed
 under the same license as the project (see LICENSE).
+
+## 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).
+  `createCommitOnBranch` signs commits only when GitHub supports signing for
+  the authenticated credential.
+- 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.