Hi, i asked this question before, it's trivial, but i need a clarification. Ok, the question is simple, as you know when time diltion is derived, we use a light clock which is perpendicular to the motion of the source(i use the terms crudely, but i think you know what i mean), that's ok we derive the time dilation formula from here and when it comes to length contraction we can use a clock that's parallel to the motion and to be consistent with the other clock the length in the direction of motion must contract. But what if we follow the derivation inversely, by considering the horizontal clock first we can not derive the contraction, because we don't know the hypotenuse effect which will come from the vertical clock, in this point we just assume that the lengths contarcts. Anyway my solution is like this, although we don't know the length contraction yet if we begin the derivation by horizontal clock, if we don't apply the contraction of length, than the time dilation equations(which we evaluate by vetical clock) get wrong. So finally length contraction occurs because of the consistency of vertical and horizontal clocks.