The four-bar mechanism, in its simplest form, has four bar-shaped links and four turning pairs, as shown in the animation. The fixed link (the black bar) may actually be bar-shaped, but more frequently it represents the frame of a machine and in that case is usually a massive casting of irregular shape.

In the four-bar mechanism, one of the rotating members usually is the driver and called as the crank or the driver ( the red bar). The other usually is called as rocker or follower (the green bar). The floating link ( the blue bar) that connects the crank and the rocker is called as connecting rod, and the fixed link ( the black bar ) is called as frame.

Many mechanism can be broken down into equivalent four-bar linkages. These mechanism has many aplication in mecanical operations. They can be considered to be one of the fundemental mechanisms.