Prime was a term of art in the discussion of alternate universes which designated whatever the speaker regarded as the "main" universe, timeline or even Earth. Thus, phrases like prime universe and Earth Prime were common to those who talked about such things. A version of Melanie Bush once thought that everyone across the many parallel universes would assume that their Earth was the "prime" one. Indeed, at least one version of the TARDIS could be programmed to travel to an "Earth Prime", which was presumably in the same universe as the "Gallifrey Prime" of that TARDIS' origin. (PROSE: Spiral Scratch)

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