Assume that the first observer uses coordinates labeled t, x, y, and z, while the second observer uses coordinates labeled t′, x′, y′, and z′. Now suppose that the first observer sees the second observer moving in the x -direction at a velocity v. And suppose that the observers' coordinate axes are parallel and that they have the same origin. Then the Lorentz transformation expresses how the coordinates are related: t ′ = t − − v x / c 2 1 − − v 2 / c 2 , x ′ = x − − v t 1 − − v 2 / c 2 , y ′ = y , z ′ = z , where c is the speed of light. If two events happen at the same time in the frame of the first observer, they will have identical values of the t -coordinate. However, if they have different values of the x -coordinate (different positions in the x -direction), they will have different values of the t' coordinate, so they will happen at different times in that frame. The term that accounts for the failure of absolute simultaneity is the vx / c.