A Frame of Reference can be regarded as a ruler with identical clocks (indicated by the red boxes) at each and every point. An observer standing at point x reading time t at her specific clock is indicated by the ordered pair, [x,t]. These are the spacetime coordinates of the observer.

In Galilean/Newtonian physics, time is assumed to be universal. That is, once two identical watches are synchronized (to simultaneously read the same time), these watches will always remain synchronized, regardless of the relative motion of the watches.

Two observers on different rulers (frames) can only see each other (or their coordinates [x,t ]) if they are positioned.perpendicular to each other. The two observers at their respective points, x=0, are synchronized with each other if their watches both read t=0 at the moment they view each other along the allowable perpendicular line-of-sight.