In a four-bar linkage both endpoints of the mechanism are fixed, forming the "ground link." One hinged joint uniformly rotates around a circle and the other one NON-uniformly moves back and forth along an arc. A crank-and-rocker mechanism is obtained from the four-bar linkage by attaching a rigid triangle to the floating link (the one in the middle). The path described by the outer vertex of this triangle heavily depends on its shape, as you can see by varying the triangle's side lengths.

Pafnuty Lvovich Chebyshev, the great Russian mathematician (1821-1894), who made a substantial contribution to numerous areas of mathematics, invented many interesting mechanisms based on the four-bar linkage. The "Lambda mechanism," called so because it resembles the Greek letter lambda, was intended to convert the rotational motion into rectilinear motion. Using his Lambda mechanism, Pafnuty Chebyshev constructed the "Plantigrade machine", which was successfully demonstrated at the Exposition Universelle in Paris in 1878 and became the prototype of modern "plantigrade" robots.