I have had two things pointed out to me repeatedly: the first as in the label for this thread and that measurements in one frame are not necessarily equal to similar measurements in another frame; i.e. that the length of a metre measured in one frame is not the same as the length in another frame. Yet, if the speed of light is the same in all frames then the length of a metre, which is defined according to the speed of light and a second is the same in every frame, then surely the metre, and all other measurements must be identical too? That is not to say that a proper length as measured in a frame at rest, is the same as a coordinate metre in a moving frame, but that the proper metre, measured at the origin of any frame at rest must be equal to a proper metre measured in the same way in any other frame. Or am I misunderstanding the terms here?