We use the standard inertial frames S and S
0 which are set up such that the x and x
0 axes coincide and
the y and y
0 axes and z and z
0 axes are parallel. Seen from S, S
0 moves in the positive x-direction with speed
v and, seen from S
0
, S moves in the negative x
0
-direction with speed v. Furthermore, it is imagined that in
each inertial frame there is an infinite set of recording clocks at rest in the frame and synchronized with each
other. Clocks in both frames are set to zero when the origins O and O0
coincide.
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