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Ponderable objects moving in free space according to Newton's First Lawconstitute both rulers and clocks when one such object is viewed from the restframe of another. Together with the Reciprocity Principle this is used todemonstrate, in both Galilean and special relativity, the invariance of themeasured length of a ruler in motion. The different times: `proper', `improper'and `apparent' appearing in different formulations of the relativistic timedilatation relation are discussed and exemplified by experimental applications.A non-intuitive `length expansion' effect predicted by the ReciprocityPrinciple as a necessary consequence of time dilatation is pointed out

Time and motion in physics: the Reciprocity Principle, relativistic invariance of the lengths of rulers and time dilatation