 Figure 5: Each of these families of straight lines, \(F_1\) and \(F_2\), represents the trajectories of a family of free particles that are relatively at rest, and therefore each defines an inertial frame. Relative to each other, the frames defined by \(F_1\) and \(F_2\) are in uniform motion.  Each of the surfaces S is a “hypersurface of absolute simultaneity” representing all of space at a given moment; evidently (given the Galilean transformations) two inertial frames will agree on which events in space-time are simultaneous.