Earth-One

The first Earth-One was given its name in Justice League of America #21 (August 1963), after The Flash #123 (September 1961) explained how Golden Age (Earth-Two) versions of characters such as the Flash (Jay Garrick) could appear in stories with their Silver Age counterparts (Barry Allen).

[2] Characters from DC Comics were originally suggestive of each existing in their own world, as superheroes never encountered each other.

That would change when the existence of another reliable Earth was established in a story titled "Flash of Two Worlds"[3][4] in which Barry Allen, the modern Flash later referred to as Earth-One (the setting of the Silver Age stories) first travels to another Earth, accidentally vibrating at just the right speed to appear on Earth-Two, where he meets Jay Garrick, his Earth-Two counterpart.

Crisis on Infinite Earths (1985–1986) was an effort by DC Comics to clean up their continuity, resulting in the multiple universes, including that of Earth-One, combining into one.

In the final issue of the 52 weekly series, it is revealed that fifty-two duplicate worlds have been created and all but New Earth have been altered from the original incarnation.