The paper first discusses the significance of the meaning of space-time coordinates as attached to “events” in the Lorentzian transformation. Restricted relativity involves two distinct parts: A—the invariance of the forms of the laws under the transformation: B—the hypothesis that similar experiments performed in relatively moving frames S and S′ give identical results. The test of A is a matter of pen and paper. B involves the hypothesis that the instruments are such that the actual measurements of space-time coordinates of events shall be related, for the two systems, by the transformation. Only by the postulation of a theory such as the quantum theory, but one relativistically invariant in sense A, can one understand the relationship between the instruments, whether the said instruments be constructed by independent observers in S and S′ from the material around them, or whether the observer in S′ acquires his instruments from S by setting them in motion. The second method of acquiring the instruments will not, in all cases, yield measurements related by the transformation. Thus, if we start with a system of isolated clocks which have been synchronized in S according to Einstein's principle, and if we transfer them to S′, their “rates” (in a suitably defined manner) may be expected to alter in accordance with the transformation. However, it will remain for the observer in S′ to synchronize the clocks in that new frame, having adjusted the time origin of one of them.