diff --git a/doc/whatis.texi b/doc/whatis.texi
index 361a3a0a1..800976af4 100644
--- a/doc/whatis.texi
+++ b/doc/whatis.texi
@@ -46,6 +46,14 @@ services can authenticate each other.
 @end macro
 @end ifhtml
 
+@ifdocbook
+@macro sub{arg}
+@docbook
+<subscript>\arg\</subscript>
+@end docbook
+@end macro
+@end ifdocbook
+
 @quotation
 @strong{Note} This discussion is about Kerberos version 4, but version
 5 works similarly.