diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 956bd83fb2..179f35d3bf 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -81,3 +81,5 @@ Basically, here's what a Git commit message should look like:
     nicely even when it's indented.
 
     Further paragraphs come after blank lines.
+
+Please keep the header line short, no more than 45 characters.