In this figure, both clocks, showing the proper time, coincided spatially at the origin of my frame, at (0, 0), and are travelling up the page in their reference frames in the time direction. Strictly, I suppose, they should have complete reference frames travelling in the time direction, within a fixed spacetime coordinate frame, but I think that, if one imagines that the clocks themselves are representing such frames, the illustration will be clear enough.

The above figure does not display a full description of the relationship of the inertial reference frames, as it does not display the Lorentz-Fitzgerald contraction effect and illustrates, immediately, the time relationships of two clocks only.

My reference frame is used as a spacetime coordinate frame in which my twin's clock is moving in space and time, and mine is moving in the time direction only, and both clocks spatially coincide at (ds, dt') = (0, 0), at the coordinate frame origin. The time relationships shown refer to these two clocks only and not necessarily to other clocks in my twin's reference frame.

If, for example, my twin has an array of spatially distributed, synchronised, clocks in his reference frame, the above figure does not imply that all of them will show synchronised observable times. As is known in Special Relativity, events simultaneous to a moving observer will not appear simultaneous in a coordinate frame in which his frame is moving.

However, if my frame also has an array of spatially distributed, synchronised clocks, the time relationships shown in the illustration will apply in respect of the locations of clocks in the two arrays which were spatially coincident when the two clocks in the illustration were spatially coincident, if such clocks are observed at those locations.

Thus, if I look at another clock in my twin's frame which was at (s, 0) from my position when I was at the origin, and is at (s+ds, dt') at the time shown in the illustration, I cannot say that I will see such a clock also showing the same observable time d', when viewed from where I am. What time it will show is a matter for a separate argument, not included here.

The figure thus refers only to the clocks actually shown, but could also be transferred to the locations of other synchronised clocks that were spatially coincident at the same time that these were.