  Separate constituents of extended systems measure proper-times on different
world-lines. Relating and comparing proper-time measurements along any two such
world-lines requires that common simultaneity be possible, which in turn
implies that the system is linearly-rigidly moving so that momentary rest
frames are identifiable at any stage of the system's journey in space-time.
  Once momentary rest-frames have been identified, clocks moving on separate
world-lines are synchronizable by light-signal communication. The
synchronization relations for two clocks are explicitly computed using
light-signals exchanged between them. Implications for the clock hypothesis are
included. Also, since simultaneity is frame-dependent, incorrect usage of it
leads to pseudo-paradoxes. Counter-examples are discussed.
