The images have been taken from page no.74,75 and 76, special relativity , A.P.French,1968 I understood that A1' and C1' are not simultaneous in S -frame. But I don't understand the principle on whose basis it is claimed that A1' and C1' are simultaneous in S' -frame. Here, the simultaneity is defined by the line A1 C1 in S -frame. I didn't understand this ,too. The simultaneity should be defined by two points A1 and C1 in S -frame. Shouldn't it? Can anyone please explain these two points?