 On the first leg of the trip, the second twin moves to the right (black sloped line); and on the second leg, back to the left.Blue lines show the "planes of simultaneity" for the traveling twin during the first leg of the journey; red lines, during the second leg.Just before turnaround, the traveling twin calculates the age of the Earth-based twin by measuring the interval along the vertical axis from the origin to the upper blue line.