The physics of space and time III: Classification of space-time   experiments and the twin paradox
  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. 
J. H. Field