Consider the reversed configuration, namely, frame Σ moves with velocity − u relative to frame Σ′, and frame Σ′, in turn, moves with velocity − v relative to frame Σ′′. In short, u → − u and v → − v by EPVR. Then the velocity of Σ relative to Σ′′ is (− v) ⊕ (− u) ≡ − v ⊕ u. By EPVR again, the velocity of Σ′′ relative to Σ is then w i = v ⊕ u. (A)