A cool diagram from the Washington Post (sorry, if I knew how to do the click to enlarge deal, I would. Click on the link below for a larger image. )
[img width=347 height=800]http://media3.washingtonpost.com/wp-dyn/content/graphic/2009/02/01/GR2009020100154.gif[/img]
http://www.washingtonpost.com/wp-dyn...020100154.html