We define a Watt quadrilateral to be a quadrilateral with a pair of opposite sides of equal length. See Figure 1.2. LinkagesThe Watt linkage (Figure 2) has equal-length cranks AD and BC, A and B fixed, and coupler bar CD. It was devised by James Watt about 1784 to constrain the steam-engine piston rod, connected at E, the midpoint of CD, to approximate straight-line motion over a limited range.