Silly question, I assume, but I can't quite figure it out. I've just started reading up on Special Theory of Relativity, and only have knowledge about Lorentz Transformations. Anyway, supposing a rod is at rest in the frame S', which is moving with velocity v with respect to frame S. Then the length of the rod in frame S' is the true length L(0), and is given by x2' - x1', where x2' and x1' are the end points of the rod in frame S'. Now we have to find the length of the rod in frame S, which is given by x2 - x1, where x2 and x1 are the end points of the rod in frame S. x2 is related to x2' by x2 = γ(x2' - vt) and x1 is related to x1' by x1 = γ(x1' - vt) Then length in frame S = x2 - x1 = γ(x2' - x1') Or length L = γL(0) Hence length to an observer in frame S is elongated. May I know where I went wrong?