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Disjointed Rollovers with an Image Map

Can someone point me in the direction of an explanation on how to do this:

How can I combine disjointed rollovers with an image map?  I'm using XHTML 1.0 Transitional.  Basically I have an image of a city and then another image that's a sign and when you hover on each building, its description appears on the sign in the 2nd image.

I also asked about this in the webdesign community.  Thanks for any help anyone can provide.