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