Abstract :
Our beautifully conceived International System of Units (S.I.) has undergone many changes since its inception, but always subject to the requirement that the measured value of any unchanging quantity such as a physical constant, shall remain unaltered. The uncertainty of these values has, however, decreased by a thousand times or more and the reproducibility and coherence of the S.I. must keep ahead. Some examples will illustrate that this is being achieved, perhaps surprisingly, by conceptually simpler methods - a philosophy which should be relevant to all kinds of measurement.
