The link connecting the driving crank with the follower crank in a four bar linkage is called the coupler. Similarly, in the case of a single slider crank mechanism the connecting rod is the coupler. During the motion of the mechanism any point attached to the coupler generates some path with respect to the fixed link. This path

is called the coupler curve. The point, which generates the path is variously called the coupler point, trace point, tracing point, or tracer point.

An example of the coupler curves generated by different coupler points is given in figure below. Mechanisms can be designed to generate any curve.