diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index db0f47aa..654127ef 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -151,6 +151,51 @@ Tests are required for:
 - New plugin hooks (contract test)
 - Schema migrations (before/after data integrity)
 
+## Branches: what belongs on main, and what does not
+
+Two branches. `main` is the product every site runs and the publication mirror
+exports. `<site>` branches - currently `wj` - hold work that is not ready for
+other sites to see.
+
+**Most work is product.** ADR-015 gives site differences three homes on `main`:
+a setting with a neutral default, seed data, or a site-namespaced directory
+(`scripts/site_imports/<site>/`, which the style gate exempts). That is the only
+site-specific home in the tree, so a site branch is about TIMING, not locality:
+things that are not ready yet, rather than things that are permanently ours.
+
+Say which at commit time, with a trailer, because the decision does not survive
+being deferred - months later it is a list of subject lines and a guess:
+
+```bash
+git commit -m "Add the thing" -m "Scope: product"   # main should have it
+git commit -m "Add the thing" -m "Scope: site"      # this site alone
+```
+
+Then:
+
+```bash
+tools/wj-status.sh                          # what is unpromoted, and where it belongs
+tools/promote-to-main.sh --dry-run <sha>    # what files would reach main
+tools/promote-to-main.sh <sha> [<sha> ...]  # oldest first; runs main's gates
+tools/git-push.sh main                      # you push, deliberately
+```
+
+Windows equivalents are `tools\git-push.cmd` and `tools\promote-to-main.cmd`.
+
+Promotion cherry-picks NAMED commits rather than merging the branch. Merging
+takes the site work along with the product work, which is how a site branch
+quietly becomes the product.
+
+**The failure to watch for is not what you think.** Promoting something
+site-specific by accident is caught by the ADR-015 gate on the way into `main`.
+The real risk is product work sitting on a site branch forever because nobody
+decided - which is how the `wilm` branch stopped being a merge and became a
+port, costing a MySQL dialect fix, a contract violation and a plugin install to
+bring back.
+
+So rebase the site branch on `main` often, and promote as you go rather than in
+one batch at the end.
+
 ## Code review checklist
 
 - [ ] No underscores in table or column names