English: Animation illustrates the time difference between twins in the twin paradox. The black line is the world line of the stationary (or uniformly moving) twin. The red line is the world line of the accelerating and decelerating twin. The momentarily frame is co-moving along the world line of the accelerating and decelerating twin. The numbers show clocks of the both twins. The clocks start simultaneously. When the twins meet again, the red clock (moving nonuniformly) is delayed compared to the black one (moving uniformly). The vertical direction indicates time, while the horizontal indicates distance, the dashed line is the spacetime of the observer. The small dots are specific events in spacetime. Based on https://en.wikipedia.org/wiki/File:Lorentz_transform_of_world_line.gif