In kinematics, Watt's linkage (also known as the parallel linkage) is a type of mechanical linkage invented by James Watt in which the central moving point of the linkage is constrained to travel on a nearly straight line. It was described in Watt's patent specification of 1784 for the Watt steam engine. Today it is used in automobile suspensions, allowing the axle of a vehicle to travel vertically while preventing sideways motion. Description Watt's linkage consists of three bars bolted together in a chain. The chain of bars consists of two end bars and a middle bar. The middle bar is bolted at each of its ends to one of the ends of each outer bar. The two outer bars are of equal length, and are longer than the middle bar. The three bars can pivot around the two bolts. The outer endpoints of the long bars are fixed in place relative to each other, but otherwise the three bars are free to pivot around the two joints where they meet. In linkage analysis there is an imaginary fixed-length bar connecting between the outer endpoints. Thus Watt's linkage is an example of a four-bar linkage. History Its genesis is contained in a letter Watt wrote to Matthew Boulton in June 1784. This type of linkage is one of several types described in Watt's 28 April 1784 patent specification. However, in his letter to Boulton he was actually describing a development of the linkage which was not included in the patent. The slightly later design, called a parallel motion linkage, led to a more convenient space-saving design which was actually used in his reciprocating, and his rotary, beam engines. The context of Watt's innovation has been described by C. G. Gibson: During the Industrial Revolution, mechanisms for converting rotary into linear motion were widely adopted in industrial and mining machinery, locomotives and metering devices. Such devices had to combine engineering simplicity with a high degree of accuracy, and the ability to operate at speed for lengthy periods. For many purposes approximate linear motion is an acceptable substitute for exact linear motion. Perhaps the best known example is the Watt four bar linkage, invented by the Scottish engineer James Watt in 1784. Shape traced by the linkage This linkage does not generate a true straight line motion, and indeed Watt did not claim it did so. Rather, it traces out Watt's curve,