Let me see if my interpretation of DOOMSDAY CLOCK's delineation of Pre-COIE continuity is correct.

Originally, there was only one universe...the DCU of the Golden Age, as it was published from 1938 to perhaps as late as 1956.

In 1956, the Multiverse was born the moment that the lightning bolt struck Central City police scientist Barry Allen, making him the Flash. That was the moment that the original universe divided into Earth-One (Silver and Bronze Age) and Earth-Two (Golden Age), though no word on the creation of other parallel universes such as Earth-Three, Earth-S, Earth-X, etc.

After COIE, Earth-One was re-created as Earth-1985, which is separate from the Post-COIE Earth whose events were chronicled in most of the DC comics published between 1986 and 2011.

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