Concept popularized between the years
1956 and
1985 in
DC Comics as an explanation for how they could team-up all their
superheroes and still retain some resemblance of
continuity for their readership.
In the comic book Showcase #4 printed in 1956, they showed Barry Allen, looking at a copy of the old Jay Garrick comic book The Flash. In Barry Allen's world, Jay Garrick was a fictional character in a kid's publication. By pure chance, Allen ends up getting similar powers, and inspired by the fictional tale, he decides to become The Flash for real. So a character is revitalized and reborn. This won over younger generations of readers, but those who remembered the Jay Garrick Flash of the 1940s wanted to see him again. So Julie Schwartz and Gardner Fox wrote "Flash of Two Worlds" where Barry Allen vibrated his molecules so fast that he slipped out of his own reality and into Jay Garrick's. In this story they establish that there are at least two different co-existing alternate realities.
This set off a chain reaction. It escalated. Not only were there two worlds. There were an infinite amount of them. Any writer could write a story where anybody in the DC Multiverse could meet anybody else. They'd just have to integrate a time machine, a vibrational distortion whatchamacallit and a couple other things and before you knew it, there'd be another fun, exciting story for comic book fans everywhere. But it just didn't make sense.
If the adventures from Earth-2's Flash had been published as comic book fiction in Earth-1, there were two Supermans, one from each world. Were his stories also published? If not, why not? If so, how come nobody knew that Superman was really Clark Kent? And that's just one of many complications and paradoxes. It gets more confusing. Because at the time the comic books being published were about the Silver Age superheroes, they were known as being of Earth-1. However the Golden Age superheroes who were older and had been around longer were dubbed inhabitants of Earth-2.
So Flash II is from Earth I and Flash I is from Earth II. Dizzy yet? It gets better.
- Earth Prime - Originally Where you and I come from. Where all these superheroes aren't real. They're fictional characters in a comic book. However, when they decided to use this world in their storytelling beyond just referring to it, it became a fictional world in which eventually, thermonuclear war occurred. So it's definitely not our Earth anymore. But it's kind of symbolic: the theory is our 'normal superhero-less' reality is out there somewhere.
- Earth-1 - The superheroes of the Silver Age of comic books, published from 1956 to 1985. They include Superman, Batman, Wonder Woman, Green Lantern, Green Arrow, and a number of superheroes that exist simultaneously in other realities. Most of these superheroes are about the same age, depicted for thirty years as being about thirty years old; or somewhere between college graduation and middle-age. They also include Superboy and Martian Manhunter, and a bunch of characters that only exist in this one Earth. This Earth also included Barry Allen, the Silver Age Flash. Many of these superheroes banded together as a team called the Justice League. Mundane history was largely identical to that of Earth Prime, except for the fact that unlike here, costumed vigilantes happened to show up in the late 1950s but didn't happen to remarkably affect any major historical events (like the Kennedy Assassination or the Space Race for examples).
- Earth-2 - Known to comic book collectors as
The Golden Age. Why? Cuz this was when costumed vigilantism in pulp comics was at its height. Instead of the Justice League, this reality had the Justice Society of America. They had a duplicate Superman, Batman, Wonder Woman, Green Lantern, as well as Jay Garrick as the original Flash. There were also an amazing collection of superheroes who were largely indiginous to this reality, most of which were at their prime before, during and after World War Two. So most of these guys were twenty to forty years older than their Earth-1 counterparts. The mundane history was largely unchanged except for the fact costumed vigilantes began proliferating in the 1940s, and in the 1970s Africa became free, whereas in Earth-1 and Earth-Prime that didn't happen until many years later. Very few comic books that took place in this reality were published in the 80s. All-Star Squadron and Infinity Inc. are the most noteworthy. Most of the titles that were published in the 1940s had been cancelled by 1956, which is what started this whole thing in the first place.
- Earth-3 - Once things got cooking, the writers threw in this world to give the heroes worthy adversaries. This Earth consisted not of a Justice League, but a Crime Syndicate with Ultraman instead of Superman, Johnny Quick instead of The Flash, Owlman instead of Batman, the list goes on. This distorted mirror shape reality also happened to have one superhero: Lex Luthor.
- Earth-4 - Created largely for legal reasons. Somewhere between 1950 and 1985, DC Comics had legally purchased or aquired the rights to all the Charlton Comics characters. Though these guys existed in an alternate reality from the DC comics, this Multiverse idea offered them the opportunity to introduce them, and turn the characters into potential commodities again. Superheroes from this world included Blue Beetle, Peacemaker, Thunderbolt, Captain Atom, Nightshade, and many more. There were no counterparts of these characters in other known realities. Although that's probably cuz, well, Charlton comics folded and DC sat on them for so many years. No opportunity to develop and improve the characters.
- Earth-S - Around the same time it bought Chartlon, DC Comics had gotten its hands on Fawcett Comics which had been the company that owned the original Captain Marvel aka Billy Batson. They had attempted to bring Shazam! back in the early 70s, but due to legalities with Marvel Comics Group, and the fact that Superman and Captain Marvel were seen as competing titles even though they were now owned by the same company, well, for a long time they just sat on them.
- Earth-5 - Technically there is no Earth Five, because 5 and S look so much alike.
- Earth-X - DC Comics treated competing comic book companies going bankrupt like they were garage sales. This time it was Quality Comics; including The Ray, Uncle Sam and the Freedom Fighters as well as other characters. What makes this reality stand out is that World War Two kept going for over thirty years after it stopped in Earth-Prime and other realities.
- Earth-6 - Made specifically for the purpose of Crisis on Infinite Earths, this Earth made an appearance showing only three superheroes who had used their powers to create a monarchy. Lord Volt and Princess Fern died soon after we learn about them, but Lady Quark survives and is rescued to the new timeline, where she promptly vanished because she was very unmarketable after Crisis was over.
- Earth-Sigma - After Crisis, five worlds were merged together to save everybody. Earths 1, 2, 4, S and X are turned into Sigma, then they blow this reality up three or four years later.
- Still others - Earth-C which was home to Captain Carrot and his Amazing Zoo Crew which strangely enough didn't appear in the Crisis on Infinite Earths at all. Earth-K which featured Kamandi the last boy in the world. Earth-Omega which was the first to be destroyed in the Crisis on Infinite Earths and where the whining Pariah character came from. Qward which was the bad guy's evil anti-matter universe, Earth-D, , Earth-Pi, Earth-Doodoo, Eartha Kitt, It got ridiculous.
In the end this whole thing is an argument both for and against continuity. I mean those who hate the anal retentiveness of comic book continuity could use this as evidence that messing with trying to keep things straight from title to title or even comic to comic is just mind-threatening. Those who like it can look at this and go, "yeah but lookit the cool stories they wrote about it while they tried to make sense out of it!" Either way it's a winner.