diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 40edc90a6af..5ac33691c91 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -1936,8 +1936,10 @@ the end of the mode line off to the right.  Emacs can ``compress'' the
 mode line if the @code{mode-line-compact} variable is non-@code{nil}
 by turning stretches of spaces into a single space.  If this variable
 is @code{long}, this is only done when the mode line is wider than the
-currently selected window.  This variable can be buffer-local to only
-compress mode-lines in certain buffers.
+currently selected window.  (This computation is approximate, based on
+the number of characters, and not their displayed width.)  This
+variable can be buffer-local to only compress mode-lines in certain
+buffers.
 
 @node Mode Line Data
 @subsection The Data Structure of the Mode Line