diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index 4d90c77c7b..894546dd75 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -404,6 +404,15 @@ For Python scripts:
    documentation for version 2.6 does not mention this prefix, it has
    been supported since version 2.6.0.
 
+Error Messages
+
+ - Do not end error messages with a full stop.
+
+ - Do not capitalize ("unable to open %s", not "Unable to open %s")
+
+ - Say what the error is first ("cannot open %s", not "%s: cannot open")
+
+
 Writing Documentation:
 
  Most (if not all) of the documentation pages are written in the