  The concepts of primary and reciprocal experiments and base and travelling
frames in special relativity are concisely described and applied to several
different space-time experiments. These include Einstein's train/embankment
thought experiment and a related thought experiment, due to Sartori, involving
two trains in parallel motion with different speeds. Spatially separated clocks
which are synchronised in their common proper frame are shown to be so in all
inertial frames and their spatial separation to be Lorentz invariant. The
interpretions given by Einstein and Sartori of their experiments, as well as
those given by the present author in previous papers, are shown to be
erroneous.
