On the left is the original green surface, with the M and N directions highlighted in in upper left corner.
On the right is the same physical surface, split into two computational surfaces. The grid lines in the left part of this surface are red, with the M and N directions highlighted in in upper left corner. The grid lines in the right part of this surface are blue, with the M and N directions highlighted in in upper left corner.
Last updated 26 Mar 2003