Another mechanism which solves the problem of rectilinear motion was described by Hart in 1874. It is based on a "intertwined parallelogram" shown left, where AB = CD, AD = BC .
If O, P, Q are three fixed points on the AB, AD and BC rods, positioned so that the straight line on which they lie is parallel to AC, the same will happen in whatever position of the mechanism, and we will have OP · OQ = BQ · QC - OA · OB = constant. Thus if O is fixed, the points P and Q correspond in a circular inversion and, if P moves on a circumference passing through O, point Q draws a straight line.

With Hart's mechanism, the study of connecting rod mechanisms for rectilinear motion has reached the minimum complexity: it is possible to demonstrate that it is impossible to solve the problem with less than five rods. But many mechanisms have been invented for the task, including the one shown on the left, based on two double kites, trough which a candle holder can be moved and kept vertical.