The velocities u and u ′ are the velocity of some massive object. They can also be for a third inertial frame (say F ′′), in which case they must be constant. Denote either entity by X. Then X moves with velocity u relative to F, or equivalently with velocity u ′ relative to F′, in turn F′ moves with velocity v relative to F. The inverse transformations can be obtained in a similar way, or as with position coordinates exchange u and u ′, and change v to − v.