Improve documentation of how vertical-motion interprets columns.

 src/indent.c (Fvertical_motion): Doc fix.

 doc/lispref/positions.texi (Screen Lines): Clarify how columns are counted
 by vertical-motion.

4 files changed, 20 insertions, 4 deletions

diff --git a/doc/lispref/ChangeLog b/doc/lispref/ChangeLog
index c20a20409a2..4610c33796a 100644
--- a/doc/lispref/ChangeLog
+++ b/doc/lispref/ChangeLog
@@ -1,3 +1,8 @@
+2014-06-21  Eli Zaretskii  <eliz@gnu.org>
+
+        * positions.texi (Screen Lines): Clarify how columns are counted
+        by vertical-motion.
+
 2014-06-14  Eli Zaretskii  <eliz@gnu.org>
 
         * commands.texi (Accessing Mouse): Improve the wording of the
diff --git a/doc/lispref/positions.texi b/doc/lispref/positions.texi
index f83173e2038..fee36fa833d 100644
--- a/doc/lispref/positions.texi
+++ b/doc/lispref/positions.texi
@@ -500,7 +500,11 @@ is negative, it moves up instead.
 The @var{count} argument can be a cons cell, @code{(@var{cols}
 . @var{lines})}, instead of an integer.  Then the function moves by
 @var{lines} screen lines, and puts point @var{cols} columns from the
-start of that screen line.
+visual start of that screen line.  Note that @var{cols} are counted
+from the @emph{visual} start of the line; if the window is scrolled
+horizontally (@pxref{Horizontal Scrolling}), the column on which point
+will end is in addition to the number of columns by which the text is
+scrolled.
 
 The return value is the number of screen lines over which point was
 moved.  The value may be less in absolute value than @var{count} if
diff --git a/src/ChangeLog b/src/ChangeLog
index f28a6a63ec4..93a25ba2871 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,7 @@
+2014-06-21  Eli Zaretskii  <eliz@gnu.org>
+
+        * indent.c (Fvertical_motion): Doc fix.
+
 2014-06-21  Paul Eggert  <eggert@cs.ucla.edu>
 
         Port to OS X ACLs (Bug#17810).
diff --git a/src/indent.c b/src/indent.c
index dc862518924..f492461bf50 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -1944,9 +1944,12 @@ The optional second argument WINDOW specifies the window to use for
 parameters such as width, horizontal scrolling, and so on.
 The default is to use the selected window's parameters.
 
-LINES can optionally take the form (COLS . LINES), in which case
-the motion will not stop at the start of a screen line but on
-its column COLS (if such exists on that line, that is).
+LINES can optionally take the form (COLS . LINES), in which case the
+motion will not stop at the start of a screen line but COLS column
+from the visual start of the line (if such exists on that line, that
+is).  If the line is scrolled horizontally, COLS is interpreted
+visually, i.e., as addition to the columns of text beyond the left
+edge of the window.
 
 `vertical-motion' always uses the current buffer,
 regardless of which buffer is displayed in WINDOW.