The timestream is an alternate history device used in Rick Sutcliffe's fiction. It is the medium in which the various alternate earths exist, or, if one prefers, it provides the connections among them, in the manner of C. S. Lewis' wood between the worlds--a place between.
Local names for the Earths are given first, the scholarly designations second. Tirdia (world of God) is a Hibernian designation. Metans and many Hibernian scholars call it Prime. Babylon is a local designation. Many Hibernian and Metan scholars refer to it as Para. Likewise, Hibernia (Greater Ireland) is a local designation. Metan scholars call it Ortho.
The diagram shows the relative positions of the earths known to Federation and Metan scholars before September 2000, as "viewed" from within the timestream itself and described by travellers there.
The diagram is an index to further information, and all the names are links.