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 theformsof the laws under the transformation: B—the hypothesis that similar experiments performed in relatively moving framesSandS′ 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 inSandS′ from the material around them, or whether the observer inS′ acquires his instruments fromSby 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 ofisolatedclocks which have been synchronized inSaccording to Einstein's principle, and if we transfer them toS′, their “rates” (in a suitably defined manner) may be expected to alter in accordance with the transformation. However, it will remain for the observer inS′ to synchronize the clocks in that new frame, having adjusted the time origin of one of them.