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This is probably a stupid question, but I'm very confused, so any help would be much appreciated!

If there are two frames moving relative to each other, I cannot tell which is 'really moving'. If I have a time interval t in the frame S, the time interval in S' is t'=gamma x t. But if I label the frames the other way around, t=gamma x t', which isn't the same. So how do I know which frame is the prime frame?