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)}