The OTU is one of the largest and best developed 2-dimensional space settings in print.
Key words: 2-dimensional.
The setting also has had several waves of redevelopment. (Early CT, also called Prototraveller; Late CT, aka Milieu 1100; MT, aka Milieu 1116; Milieu 1125 for Hard Times ; TNE, aka Milieu 1200; T20, which is the mid ten-hundreds.)
The GURPS Traveller line is the most recent (and fairly controversial) since it provides a level of detail unprecedented in any other versions; further, it's not of necessity the OTU; The GTU was from the outset divergent from the main line.