Four bar linkage
If four bars of different lengths are linked together to form a quadrilateral structure, they make what is called a four bar linkage. The bars can be different lengths, and the term "bar" can be used to describe any rigid (stiff) body. The entire structure can change shape, depending on what forces are applied to the bars. Four bar linkages are planar, and simple mechanisms in mechanical engineering.
In general, when analyzing the motion of a four bar linkage, one of the bars is stationary. This bar is called the ground link or fixed link. (The term "link" is used as an alternate term for the bar.) The two bars attached to the ground link are called the grounded links. The remaining bar, which is opposite the ground link and not contiguous with it, is called the coupler link.
In mechanical action, one of the grounded links is usually acted on by a force, making it the input link. The other grounded link is then called the follower link, since its action is influenced by the input link.
There are many different types of four bar linkages, classified by the relative lengths of the bars.