Jill is at rest, and Zak is on a train moving with velocity such that . Jill and Zak each have a set of clocks, and in their respective rest frames the clocks all appear to be set at , spaced 1 m apart. At the point in spacetime shown, the middle clocks are aligned at the same location and time . You can see how Jill perceives her clocks compared to Zak's clock and vice versa.