Here is a URL that explains it, but I ;haven't read it yet, and expect difficulty making
useful post about it.
http://www.geoshare.org/new_web_site/technical/12/html/1.htm

P.S. Two hours later, I went back and tried to understand that URL site. As I feared my smarts
have deteriorated to point I cannot master it. My respects to those of you who can.