in mechanical engineering, a system of solid, usually metallic, links (bars) connected to two or more other links by pin joints (hinges), sliding joints, or ball-and-socket joints so as to form a closed chain or a series of closed chains. When one of the links is fixed, the possible movements of the other links relative to the fixed link and to one another will depend on the number of links and the number and types of joints. With four pin-connected links, for example, the links all move in parallel planes, and regardless of which link is fixed, the other links have constrained motion; i.e., they move in a fixed and determinate way relative to the fixed link. By varying the relative lengths of the links, this four-bar linkage becomes a useful mechanism for converting uniform rotary to non-uniform rotary motion or continuous rotary to oscillatory motion; it is the most commonly used linkage mechanism in machine construction.