The simplest moveable closed-chain connection is a four-bar connection, sometimes called a four-bar connection. It is composed of four bodies, called bars or ties, linked by four joints in a circle. The joints are usually arranged so that the ties travel in parallel planes, and the arrangement is called a four-bar planar attachment.

A four-bar linkage, also called a four-bar, is the simplest movable closed chain linkage. It consists of four bodies, called bars or links, connected in a loop by four joints. Generally, the joints are configured so the links move in parallel planes, and the assembly is called a planar four-bar linkage. Spherical and spatial four-bar linkages also exist and are used in practice.