Fig. 2-3a redraws Fig. 2-2 in a different orientation. Fig. 2-3b illustrates a relativistic spacetime diagram from the viewpoint of observer O. Since S and S′ are in standard configuration, their origins coincide at times t = 0 in frame S and t ′ = 0 in frame S′. The ct ′ axis passes through the events in frame S′ which have x ′ = 0. But the points with x ′ = 0 are moving in the x -direction of frame S with velocity v, so that they are not coincident with the ct axis at any time other than zero. Therefore, the ct ′ axis is tilted with respect to the ct axis by an angle θ given by