Figure 4. The S' reference frame has a velocity v in the positive X axis direction, and the S reference frame is at rest. At t = 0, the S origin, S' origin, eventon A, and eventon B all coincide. Eventon A moves on a straight path, crossing ct distance from (0, 0) to point (x, y) relative to S; and eventon A moves along the Y' axis, crossing D' distance from (0', 0') to (0', y') relative to S'. Eventon B moves along the X' axis, crossing D' distance relative to S'.