The Einstein law of velocity addition is well-known. Consider three inertial reference frames S, K, and M, with X axes parallel in the same sense. Let K have velocity v1 along the X axis of S, and M have velocity v2 along the X axis of K. Then, according to the Einstein formula, the velocity v3 of M along the X axis of S is If v2 is the velocity of an object at rest in M as measured by an observer in K, then v3 is the velocity of the same object as measured by an observer in S. The formula is valid for a change in reference frame of the observer.