Suppose two bodies are connected so that whenever one moves a certain amount, the other must move by the same amount, although not necessarily in the same direction.  For instance, two blocks hang from an inelastic cord that runs over a pulley would be called "connected bodies."  In this case the velocities of the two bodies have the same magnitude.

A special case of this principle applies to two rotating objects coupled by a belt which moves without slipping. In this case, the the points on the rims of the respective objects which are in contact with the belt are constrained to have the same speed. That is, the linear velocities of these two points will have the same magnitude, even if the rotational speeds of the two rotating objects are different.