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