Four bar linkages convert rotary motion to reciprocating motion or apply mechanical advantage. This mechanism scales the motion by a factor of two, and scales the torque by a factor of 1/2.

In this example, the lengths of the linkages are equal. The motor driver the pantograph mechanism in a circle (pink), and the output is a circle with twice the radius.

By altering the lengths of one of the linkages, an output shape is not circular. In this example, the length of the white link is 10% shorter than in a parallelogram. The resulting output shape is more of an oval.