The transformations become more complicated for a chain of attached rigid bodies. For convenience, each rigid body is referred to as a link. Let , , ..., denote a set of links. For each such that , link is ``attached'' to link in a way that allows some constrained motion with respect to . The motion constraint must be explicitly given, and will be discussed shortly. As an example, imagine a trailer that is attached to the back of a car by a hitch that allows the trailer to rotate with respect to the car. In general, a set of attached bodies will be referred to as a linkage. This section considers bodies that are attached in a single chain. This leads to a particular linkage called a kinematic chain.