1 files changed, 5 insertions, 1 deletions

diff --git a/src/dispextern.h b/src/dispextern.h
index 5dde56f6118..87946cde963 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -1241,7 +1241,11 @@ struct face
   unsigned int pixmap_w, pixmap_h;
   
   /* Non-zero means characters in this face have a box that thickness
-     around them.  */
+     around them.  If it is negative, the absolute value indicates the
+     thickness, and the horizontal lines of box (top and bottom) are
+     drawn inside of characters glyph area.  The vartical lines of box
+     (left and right) are drawn as the same way as the case that this
+     value is positive.  */
   int box_line_width;
 
   /* Type of box drawn.  A value of FACE_NO_BOX means no box is drawn