Originally Posted by
Jeff Wills
In deference to Sheldon's ghost, I've refered to it as a "derailing thingy". So there. Nyah, nyahhh....
Thank you Mr. Wills for drawing my attention to another well written artical by the late Sheldon Brown:
http://www.sheldonbrown.com/derailer.html
I now will also spell the name for the "derailing thingy" as derailer as the late Sheldon Brown asked us to.