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.PACS 03.30.+p which he passes by the two ends of the train and calculates its length with Eqn(2.1). This method is also applicable when both the observer and the measured train are in motion with known speeds, when an appropriate velocity addition formula is used to calculate the speed, v, to be substituted in (2.1).The fifth method involves observation, in the frame S, of the time difference of spatial coincidences of the front end of the moving train, with rest frame S', used in the fourth method, with the back and front ends of the stationary to-be-measured train. Again the length of the train is given by Eq. (2.1).Suppose now that two independent measurements of the length of a train are made, one using the fourth and the other the fifth of the methods just described. If different results are obtained for the length of the train, three different causes, in one-to-one correspondence with the quantities in Eqn(2.1), may be cited:(i) The train changed length between the two measurements.(ii) The observer's train, was moving at a different speed during the two measurements.(iii) The clocks employed in the two measurements, one in the frame S, the other in the frame S', are running relatively fast (or slow) with respect to each other.