Before Crisis on Infinite Earths, DC continuity included a multiverse of parallel Earths, including Earth-One, Earth-Two, and Earth-Three. After Crisis on Infinite Earths occurred, all Earths were collapsed into one, dubbed by fans as Earth-Only. Zero Hour further refined that collapse, which hypertime promptly circumvented by stating that the Crisis had created one timeline in which all parallel Earths had collapsed into one but the other timelines in which they had not still existed.

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