Let there be two inertial frames of reference, S and S', constructed so that the axes of S and S' coincide at t = t' = 0, where t is the time as measured in S and t' is the time as measured in S'.
See velocity-addition formula, where \beta = v/c and \gamma = (1 - \beta^2)^{-\frac{1}{2}}, and c is the speed of light in a vacuum.