  A nomenclature for inertial frames and a notation for space and time
coordinates is proposed to give an unambigous description of space-time
experiments in special relativity. Of particular importance are the concepts of
`base' and `travelling' frames and `primary' and `reciprocal' experiments. A
detailed discussion of the twin paradox is presented. The physical basis of the
differential aging effect is found to be a relativistic relative-velocity
transformation relation, not, as hitherto supposed, the spurious `length
contraction' effect.
