The Hart linkage (of type 2) is planar linkage consisting only of revolute joints, that is capable of transforming rotary motion into perfect straight-line motion, and vice versa. The type 2 linkage is unique in that it was the first straight line mechanism with revolute joints that was not based on the theory of inversion of a circle.  

The following animation shows the mathematics behind the working principle of the Harts mechanism of type 2.