An Unearthly Child
Normal Space, usually referred to as N-Space for short, was the name given to the prime universe of Gallifrey to distinguish it from the smaller E-Space. Alternatively, it was also known as the “third universe” (Quinnis) or Home D, to distinguish it from other dimensions, like Shadow-Space. (Masquerade) Donna Nobleonce referred to this universe as The Doctor’s World. (Turn Left)
Because of entropy, N-Space should have ceased to exist as a result of heat death some time before the 20th century in the Humanian Era, if not for the Logopolitans. The Logopolitans used Block Transfer Computations to create the CVEs to vent entropy from N-Space into E-Space. (Logopolis) Sometimes the CVEs also allowed physical objects to travel from N-Space into E-Space. (Full Circle)
Unlike E-Space, which used coordinates with a negative value, relative to the navigational system of the Doctor’s TARDIS, N-Space used positive coordinates. The Fourth Doctor suggested, then, that N-Space was a positive universe and E-Space its negative counterpart.
The TARDIS scanner continued to show images of N-Space even after materialising on Alzarius, which had the same co-ordinates as Gallifrey, though reversed. The Doctor suggested that, rather than relaying visual information, the scanner operated off the absolute value of the co-ordinates and could not process negative coordinates. The scanner system was made functional by replacing its original N-Space image translator with one native to E-Space. (Full Circle)
The Gateway, whose co-ordinates had a zero value (neither positive nor negative) served as a place of transit between N-Space and E-Space. (Warriors’ Gate)
Aside from the Gateway, objects could have passed from N-Space to E-Space through a CVE, as did The Doctor’s TARDIS. (Full Circle) The Great Vampire travelled to E-Space from N-Space by means unknown, as did the human ship, the Hydrax. The Doctor surmised that the Hydrax was intentionally brought into E-Space by the Great Vampire. (State of Decay)