1 files changed, 4 insertions, 1 deletions

diff --git a/man/dired.texi b/man/dired.texi
index 761c52215fd..8bb014a59b9 100644
--- a/man/dired.texi
+++ b/man/dired.texi
@@ -786,7 +786,10 @@ regular expression specified in any @kbd{%} command as a default.
 Compare the current file (the file at point) with another file (the file
 at the mark) using the @code{diff} program (@code{dired-diff}).  The
 file at the mark is the first argument of @code{diff}, and the file at
-point is the second argument.
+point is the second argument.  Use @kbd{C-@key{SPC}}
+(@code{set-mark-command}) to set the mark at the first file's line
+(@pxref{Setting Mark}), since @code{dired-diff} ignores the files marked
+with the Dired's @kbd{m} command.
 
 @findex dired-backup-diff
 @kindex M-= @r{(Dired)}