diff --git a/README.md b/README.md
index dce7bca..135dc6c 100644
--- a/README.md
+++ b/README.md
@@ -193,7 +193,7 @@ Environments need the App's repository Administration permission, variables and
 
 `organization.rulesets` is added to every repository's `rulesets`, with the same replace, `~` opt-out and repository-only semantics as [environments](#environments).
 
-Every ruleset protects the repository's default branch: changes only through a pull request (no approval required, so a single maintainer can merge their own), no force pushes, no deletion. No required status checks. On the GitHub Free plan, rulesets are available in public repositories only.
+Every ruleset protects the repository's default branch: changes only through a pull request (no approval required, so a single maintainer can merge their own), no force pushes, no deletion, linear history, [conventional commit](https://www.conventionalcommits.org/) messages with a lowercase subject. No required status checks. On the GitHub Free plan, rulesets are available in public repositories only.
 
 `bypass_apps` lists GitHub App IDs that always bypass the ruleset, e.g. a release App pushing a changelog commit to the default branch. Pushes authenticated by `GITHUB_TOKEN` cannot bypass: a repository releasing with `GITHUB_TOKEN` must opt out.
 
diff --git a/terraform/modules/repository/main.tf b/terraform/modules/repository/main.tf
index 7d7224b..9028b38 100644
--- a/terraform/modules/repository/main.tf
+++ b/terraform/modules/repository/main.tf
@@ -75,8 +75,15 @@ resource "github_repository_ruleset" "this" {
   }
 
   rules {
-    deletion         = true
-    non_fast_forward = true
+    deletion                = true
+    non_fast_forward        = true
+    required_linear_history = true
+
+    commit_message_pattern {
+      name     = "Conventional commit, lowercase subject"
+      operator = "regex"
+      pattern  = "^(build|chore|ci|docs|feat|fix|perf|refactor|revert|style|test)(\\([a-z0-9._/-]+\\))?!?: [^A-Z\\n]+(\\n|$)"
+    }
 
     pull_request {
       required_approving_review_count = 0
diff --git a/terraform/modules/repository/tests/repository.tftest.hcl b/terraform/modules/repository/tests/repository.tftest.hcl
index 78d3912..e218e60 100644
--- a/terraform/modules/repository/tests/repository.tftest.hcl
+++ b/terraform/modules/repository/tests/repository.tftest.hcl
@@ -537,6 +537,16 @@ run "ruleset" {
     error_message = "Expected deletion and force push blocked."
   }
 
+  assert {
+    condition     = github_repository_ruleset.this["default-branch"].rules[0].required_linear_history
+    error_message = "Expected linear history required."
+  }
+
+  assert {
+    condition     = github_repository_ruleset.this["default-branch"].rules[0].commit_message_pattern[0].operator == "regex" && startswith(github_repository_ruleset.this["default-branch"].rules[0].commit_message_pattern[0].pattern, "^(build|chore|ci|docs|feat|fix|perf|refactor|revert|style|test)")
+    error_message = "Expected a conventional commit message pattern."
+  }
+
   assert {
     condition     = github_repository_ruleset.this["default-branch"].rules[0].pull_request[0].required_approving_review_count == 0 && length(github_repository_ruleset.this["default-branch"].rules[0].required_status_checks) == 0
     error_message = "Expected a pull request with no approvals and no status checks."