diff --git a/pages/common/m4.md b/pages/common/m4.md
index c1f794949..9d8c1b314 100644
--- a/pages/common/m4.md
+++ b/pages/common/m4.md
@@ -1,6 +1,7 @@
 # m4
 
 > Macro processor.
+> More information: <https://www.gnu.org/software/m4>.
 
 - Process macros in a file: