The units are pixels. When a point within the rectangle is selected the given URL is used. Circular and polygonal areas are also supported. Consult the Apache documentation for more inforamation. The information included is much like that used in the server-side map file.
Some browsers allow the usemap parameter to refer to another file.
The old instructions for the CERN server are still available for reference purposes. This page was last updated on 16 June 2000.