Designed by James Watt, Watt's linkage is a simple device which will allow almost straight-line motion of a point, given two anchor points, and only rotating links.
It may be extended to allow 2 dimensional motion, with the third dimension fixed - ie. planar motion only is possible.
The linkage appears as follows:
Pivots A and E are fixed to a solid body. B and C
are free-floating. There are certain equal distances: AB = DE and BC = CD.
In this arrangement, B and D will move in an arc relative to A and E respectively. This motion will, however, cause C to move in a straight line, with only a small amount of error, parallel to BD in its starting position.
Given a suitable AB and DE length, the error is within allowable tolerance.
The first use of Watt's linkage was around 1769, to support the piston on his beam engine.
Later, from the 1950s, high-end automotive applications, applied the linkage to prevent side to side motion of the rear transmission. Leaf-spring suspension generally allows a lot of such motion - especially when cornering at speed.
Points A and E are fixed to the bodywork, and the links are all rose jointed. With point C attached by a pivot to the differential casing, this setup allows the suspension to spring up and down, yet does not allow the system to slip sideways.
An even more advanced application applies two Watt's linkages to the prop tube, attempting to twist the tube in opposite directions on body roll. This turns the prop tube into a torsion bar - an effective anti-roll mechanism.