From: patzermike.mc@gmail.com   
      
   Well, personally I see no problem in saying that two comoving observers   
   see each other's clock as running slower.  The usual twin paradox is a   
   different situation.  The two clocks are reunited and hence one (or   
   both) clocks have to turn around and undergo acceleration.  Actually,   
   though, I think it is misleading to layman to talk about acceleration.   
   One can simply idealize the acceleration as instantaneous.  But then   
   things are not all relative.  Suppose I stay at home while my twin hops   
   into her rocket ship and does a round trip to Alpha Centauri and back at   
   relativistic speed.  Then there are THREE inertial frames involved.   
   Frame 1 is my frame on earth which is inertial ( to a good degree of   
   approximation).  Frame 2 is my twin's frame on the journey to Alpha   
   Centauri and frame 3 is my twin's frame on the return journey.  One can   
   calculate the time on the clocks in any of these frames and collate the   
   results with the relevant Lorentz transformations.  The results are   
   consistent.  My twin's clock shows less elapsed time.  But instead of   
   idealizing the turnaround as instantaneous, let us consider a physically   
   realistic worldline.  Suppose I am inertial and my twin travels on a   
   worldline (x(t),t) for a