diff --git a/changelog.d/17399.doc b/changelog.d/17399.doc
new file mode 100644
index 000000000..7a3fcf24c
--- /dev/null
+++ b/changelog.d/17399.doc
@@ -0,0 +1 @@
+Clarify that changelog content *and file extension* need to match in order for entries to merge.
diff --git a/docs/development/contributing_guide.md b/docs/development/contributing_guide.md
index 76c3e790c..f079f61b4 100644
--- a/docs/development/contributing_guide.md
+++ b/docs/development/contributing_guide.md
@@ -449,9 +449,9 @@ For example, a fix in PR #1234 would have its changelog entry in
 > The security levels of Florbs are now validated when received
 > via the `/federation/florb` endpoint. Contributed by Jane Matrix.
 
-If there are multiple pull requests involved in a single bugfix/feature/etc,
-then the content for each `changelog.d` file should be the same. Towncrier will
-merge the matching files together into a single changelog entry when we come to
+If there are multiple pull requests involved in a single bugfix/feature/etc, then the
+content for each `changelog.d` file and file extension should be the same. Towncrier
+will merge the matching files together into a single changelog entry when we come to
 release.
 
 ### How do I know what to call the changelog file before I create the PR?