Class 1: The shortest link is ground. The crank, coupler and rocker all make complete
revolutions. Note that the coupler makes a complete revolution even though no point on it traces out
a circle.
Class 2: The shortest link is the crank, and only the crank makes a complete revolution.
Class 3: The shortest link is the coupler. Only the coupler makes a full revolution, and we
must drive the coupler to actuate the linkage to its full extent of motion.
Class 4: The shortest link is the rocker, and only the rocker makes a complete revolution. In this
it is analagous to the Class 2 linkage.