Watt's 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 an approximation to a straight line. It was described in Watt's patent …
Figure 9 from James Watt's patent application (top left part) showing the straightline linkage
Watt's linkage train suspension
Hand-drawn diagram by James Watt (1808) in a letter to his son, describing how he arrived at the design.