--- manual/s_phys_pkgs/text/exch2.tex	2004/05/10 21:39:11	1.19
+++ manual/s_phys_pkgs/text/exch2.tex	2005/06/10 19:34:09	1.23
@@ -1,4 +1,4 @@
-% $Header: /home/ubuntu/mnt/e9_copy/manual/s_phys_pkgs/text/exch2.tex,v 1.19 2004/05/10 21:39:11 afe Exp $
+% $Header: /home/ubuntu/mnt/e9_copy/manual/s_phys_pkgs/text/exch2.tex,v 1.23 2005/06/10 19:34:09 afe Exp $
 % $Name:  $
 
 %%  * Introduction
@@ -215,12 +215,20 @@
 The number of tiles MITgcm allocates and how they are distributed
 between processors depends on \varlink{nPx}{nPx} and
 \varlink{nSx}{nSx}.  \varlink{nSx}{nSx} is the number of tiles per
-processor and \varlink{nPx}{nPx} is the number of processors.  The total
-number of tiles in the topology minus those listed in
-\file{blanklist.txt} must equal \code{nSx*nPx}.  Note that in order to 
+processor and \varlink{nPx}{nPx} is the number of processors.  The
+total number of tiles in the topology minus those listed in
+\file{blanklist.txt} must equal \code{nSx*nPx}.  Note that in order to
 obtain maximum usage from a given number of processors in some cases,
-this restriction might entail sharing a processor with a tile that would 
-otherwise be excluded. \\
+this restriction might entail sharing a processor with a tile that
+would otherwise be excluded because it is topographically outside of
+the domain and therefore in \file{blanklist.txt}.  For example,
+suppose you have five processors and a domain decomposition of
+thirty-six tiles that allows you to exclude seven tiles.  To evenly
+distribute the remaining twenty-nine tiles among five processors, you
+would have to run one ``dummy'' tile to make an even six tiles per
+processor.  Such dummy tiles are \emph{not} listed in
+\file{blanklist.txt}.\\
+
 
 The following is an example of \file{SIZE.h} for the twelve-tile
 configuration illustrated in figure \ref{fig:12tile} running on 
