The two original surfaces are on the left, drawn in blue and green. The bottom edge of the blue surface is located very close to the top edge of the green surface.
The same blue surface is shown on the right, but the original green surface has been replaced by a new surface, drawn in red, with the top edge moved up to abut the bottom edge of the blue surface. Except for the points on the top edge, all the other points in the new red surface are at the same location as in the original green surface.
Last updated 28 Mar 2003