# Why do we use two stationary clocks to compare the time elapsed in one moving clock?

Consider a train moving with respect to ground. Then a observer on ground says that the train clock runs slow while a observer on the train claims that the ground clock runs slow. Who is right? In order to resolve this contradiction, in many books, two stationary clocks and one moving clock to compare time are used.

Why? Why not only one in each frame?

• Both of them are right. Why does one of them have to be wrong? Everything is fine as long as the space-time interval is conserved between two events. Commented Jun 16, 2017 at 6:39

The famous two stationery clock experiment's that you are referring to is used so that you can compare the moving clock with the stationery clock when $\Delta x$ between them is 0 so that both the observers will read the same thing on them.
Different clocks which are synchronous for one observer in space are not synchronous for the other person who is travelling with some velocity because $\Delta t '$ depends on $\Delta x$.