diff --git a/doc/asdf.texinfo b/doc/asdf.texinfo
index c9195e9a293b2e4b3f527fe587da09b5d3bf6aff..c7b41e68e5f9f1e81cf4a71b3f09c828a9e53b55 100644
--- a/doc/asdf.texinfo
+++ b/doc/asdf.texinfo
@@ -3140,6 +3140,15 @@ initform for the @code{type} slot:
    ((type :initform "cl")))
 @end lisp
 
+To support both ASDF 1 and ASDF 2,
+you may omit the above @code{type} slot definition and instead define:
+
+@lisp
+(defmethod source-file-type ((f my-cl-source-file) (m module))
+  (declare (ignorable f m))
+  "cl")
+@end lisp
+
 Then make your system use this subclass in preference to the standard
 one: