===================================================================
RCS file: /cvs/mandoc/tbl.7,v
retrieving revision 1.9
retrieving revision 1.10
diff -u -p -r1.9 -r1.10
--- mandoc/tbl.7	2011/01/11 14:12:01	1.9
+++ mandoc/tbl.7	2011/01/11 14:15:56	1.10
@@ -1,4 +1,4 @@
-.\"	$Id: tbl.7,v 1.9 2011/01/11 14:12:01 kristaps Exp $
+.\"	$Id: tbl.7,v 1.10 2011/01/11 14:15:56 kristaps Exp $
 .\"
 .\" Copyright (c) 2010 Kristaps Dzonsons <kristaps@bsd.lv>
 .\"
@@ -226,6 +226,8 @@ Vertically span rows from the last non-
 .Ns Cm ^
 data cell.
 It is an error to invoke a vertical span on the first layout row.
+Unlike a horizontal spanner, you must specify an empty cell (if it not
+empty, the data is discarded) in the corresponding data cell.
 .It Cm \-
 Replace the data cell (its contents will be lost) with a single
 horizontal line.