diff --git a/doc/notfound.md b/doc/notfound.md
index 2d7fbac..c52ed52 100644
--- a/doc/notfound.md
+++ b/doc/notfound.md
@@ -2,7 +2,7 @@
 
 # Not Found
 
-The Not Found reply (421) is when the server doesn't recognise a command a client sends.
+The Not Found reply (421, ERR_UNKNOWNCOMMAND in RFC 1459) is when the server doesn't recognise a command a client sends.
 
 Say the person executing the unknown command has the nickname `nick`
 ```