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Prior to Crisis on Infinite Earths, the main Earths were Earth-1, home of the Justice League, Earth-2, home of the Justice Society, and Earth-S, home to the Shazam characters. There were other Earths like the Nazi-ruled Earth-X, sure, but these three got the most attention and had ongoing series set on those worlds. The only other Earths that kept popping up pretty regularly were Earth-3, home of the evil JLA doppelgangers, the Crime Syndicate, the cartoon animal Earth-C of Captain Carrot & The Zoo Crew, and Earth-Prime, where we live. Earth-4 was retroactively home to all the Charlton Action Heroes comics of Blue Beetle, The Question and Captain Atom.
All these Earths were rolled up into one merged world known as New Earth for the Post-Crisis era.
When the Multiverse returned following Infinite Crisis, New Earth, now re-dubbed Earth-0 continued to be the only main Earth, but all of the various Elseworlds stories of the Post-Crisis period were now assigned to the reborn Multiverse of 52 Earths. The other Earths would pop up in different stories, but Earth-0 got the vast majority of the attention because the entirety of this new Multiverse is built upon Earth-0.
After the New 52 reboot, all comics remained on Earth-0, except for the Earth-2 ongoing comic starring the rebooted JSA characters. Because Earth-0 is the keystone to the Multiverse, when its history was radically reshaped, so were the many of the 52 Earths.
Currently, all DC comics are set on Earth-0, although there will be a new monthly Freedom Fighters set on Earth-X soon.