The mass point model is not suitable when the orientation is relevant to the problem under study. The simplest model incorporating the orientation is the rigid body, which is defined as a set of material points with constant mutual distances. This definition is so close to that of the reference frame that both concepts are taken as synonyms in a kinematics context. The reference frame of points fixed with respect to the rigid body is called body reference frame. The kinematics of the rigid body corresponds to the transportation motion (Chapter 2) associated with that reference frame.