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References on...

Cauchy's theorem
Marek Kordos' page on the theorem. After all, why take our word for it?

Four-bar linkages
Four-Bar Linkage Analysis and Synthesis at the Integration Engineering Laboratory at UC Davis. Everything you could conceivably want to know about the behaviour of four-bar linkages!

A paper on a parallel motion generator using four-bar linkages. In particular, check out the neat keyboard storage mechanism application.

A very cool applet which simulates the motion of a four-bar linkage. It'll let you drag anchor points to modify the output trajectory!