What is a Linkage?

A linkage will be a mathematical model of a physical system that is made of rods which are connected at hinges or joints. The hinges are much like elbow joints, except that the two rods connected at a hinge can swing completely around. We will position linkages in a 2 dimensional plane, and some of the hinges will be fixed to an immobile location. Here is a picture of a simple linkage:



The point with an x is called an anchor point. It cannot move, although the rod labeled a can swivel around the anchor. Which of the following is not a configuration (i.e., a position) of the above linkage?