Editing Special relativity (section)

=== Standard configuration ===
To gain insight into how the spacetime coordinates measured by observers in different [[Inertial frame of reference|reference frames]] compare with each other, it is useful to work with a simplified setup with frames in a ''standard configuration''.<ref name="Collier">{{cite book|title=A Most Incomprehensible Thing: Notes Towards a Very Gentle Introduction to the Mathematics of Relativity|last1=Collier|first1=Peter|publisher=Incomprehensible Books|year=2017|isbn=9780957389465|edition=3rd}}</ref>{{rp|107}} With care, this allows simplification of the math with no loss of generality in the conclusions that are reached. In Fig.&nbsp;2-1, two [[Galilean reference frame]]s (i.e., conventional 3-space frames) are displayed in relative motion. Frame S belongs to a first observer ''O'', and frame {{prime|''S''}} (pronounced "S&nbsp;prime" or "S dash") belongs to a second observer {{prime|''O''}}.
* The ''x'', ''y'', ''z'' axes of frame S are oriented parallel to the respective primed axes of frame {{prime|''S''}}.
* Frame {{prime|''S''}} moves, for simplicity, in a single direction: the ''x''-direction of frame S with a constant velocity ''v'' as measured in frame ''S''.
* The origins of frames S and {{prime|S}} are coincident when time {{nowrap|1=''t'' = 0}} for frame S and {{nowrap|1={{prime|''t''}} = 0}} for frame {{prime|''S''}}.

Since there is no absolute reference frame in relativity theory, a concept of "moving" does not strictly exist, as everything may be moving with respect to some other reference frame. Instead, any two frames that move at the same speed in the same direction are said to be ''comoving''. Therefore, ''S'' and {{prime|''S''}} are not ''comoving''.