Multiverse (DC Comics)
In most of the DC Comics media, the Multiverse is a "cosmic construct" that is composed of the many universes the stories of DC media take place in. The worlds within the multiverse share a space and fate in common, and its structure has changed several times in the history of DC Comics.
History
Golden Age
The concept of a universe and a multiverse in which the fictional stories take place was loosely established during the Golden Age of Comic Books. With the publication of All-Star Comics #3 in 1940, the first crossover between characters occurred with the creation of the Justice Society of America, which presented the first superhero team with characters appearing in other publications to bring attention to less-known characters. This established the first shared "universe", as all these heroes now lived in the same world. Prior to this publication, characters from the different comic books seemingly existed in different worlds.Later, Wonder Woman #59 presented DC Comics' first story depicting a parallel "mirror" world. Wonder Woman is transported to a twin Earth where she meets Tara Terruna, who is exactly like her. Tara Terruna means "Wonder Woman" in the native language of that world. Wonder Woman describes this world as being a twin world existing alongside Earth with duplicates of everyone but with a different development. The concept of different versions of the world and its heroes was revisited in the pages of Wonder Woman a few times later.
Silver Age
Led by editor Julius Schwartz and writer Gardner Fox, DC Comics' superheroes were given a "reboot" with the publication of Showcase #4 in 1956, where a new version of the Flash made his first appearance. This is considered the beginning period of the Silver Age of Comic Books. The success of this new Flash led to the creation of new incarnations of the Golden Age characters who only shared the names and powers but had different secret identities, origins and stories. Later, new versions of other heroes, Superman, Batman and Wonder Woman, were also restarted by retelling their origins but keeping their secret identities. Fox, who worked before in the creation of the JSA, where other heroes met for the first time, created the story "Flash of Two Worlds" in The Flash #123, where Barry Allen, the new Flash, is transported to the Earth where the original Flash, Jay Garrick, existed. To Barry, Jay Garrick's world was a work of fiction as it was in the real world. This story not only presented the encounter of two worlds and the existence of the Multiverse for the first time, but it also presented key features of the Multiverse: all the universes vibrate at a specific frequency which keeps them separated; these "barriers" could be trespassed by "tuning" to that vibration. Because people could also "tune-in" these worlds in dreams, some people wrote comic books with the stories from those worlds they dreamed, which explained why Barry knew about Jay as a fictional character.The success of this story led to the first team crossover between the new Justice League of America and the Golden Age JSA, in the stories "Crisis on Earth-One" and "Crisis on Earth-Two", published in late 1963. This story arc started the tradition of a yearly crossover between the JLA and the JSA, and established the concept of a Multiverse and the designation of names, Earth-One being the JLA reality and Earth-Two the JSA reality. The success of these crossovers spawned publications telling the further stories of the Golden Age heroes in the present-day parting from many of the stories told, thus establishing a more defined continuity for every universe.
This concept of parallel Earths with differences in locations, persons and historical events became a very important ingredient within DC Comics' publications. It helped to explain continuity errors, to retell and retcon stories, and to incorporate foreign elements that could actively interact with everything else and allow them to have an "existence". Continuity flaws between the established Earth-Two and several stories from the Golden Age were given separate Earths. "Imaginary" stories and sometime divergences of Earth-One were given also separate realities. In addition to the stories appearing mainly in the pages of JLA that created new Earths, the acquisition of other comic book companies and characters by DC Comics incorporated these new properties as Earths in the Multiverse which would interact several times with the "main" Earths, One and Two; two of the most notable being Earth-S for the Fawcett Comics characters, and Earth-X for the Quality Comics characters. By the 1970s, everything that was published or related officially to DC Comics' titles could become part of the Multiverse, although much of it remained largely uncatalogued.
The names of the worlds were usually in the format Earth, hyphen, spelled numeral/letter/name. In the case of worlds with numerals, the "rule" of spelling the number was not always followed, even within the pages of the same issue.
''Crisis on Infinite Earths''
As the 50th anniversary of DC Comics drew near, major events were proposed for the celebration: an encyclopedia the Black Canary of Earth-One being the daughter of the original Black Canary, who was a resident of Earth-Two, and 2) the existence of Golden Age comic books on Earth-One and the people not noticing that some of the characters in those comic books existed in "real life". In addition, many universes had multiple alternate timelines, such as Kamandi and the Legion of Super-Heroes both being from Earth-One.Writer Marv Wolfman took this crossover event as an opportunity to reform all the fictional universe of DC Comics to avoid further continuity errors and update the DC characters to modern times. The whole Multiverse is destroyed except for five Earths. Later, the universe is recreated as one single universe with elements of the surviving five.
The crossover event Convergence officially retconned the events of Crisis after heroes in that series went back in time to prevent the collapse of the Multiverse.
Modern Age
DC Comics
After the conclusion of Crisis on Infinite Earths, the concept of a single Universe containing most elements of the "surviving Earths" was established and heavily enforced to avoid the continuity problems of the Multiverse. However, alternate realities that affected the new DC Universe appeared very quickly. In Superman #8, a Universe inside the Universe was revealed to have been created to preserve the Legion of Super Heroes' 30th century in New Earth. This world was used to allow crossovers with certain characters of the Legion of Super-Heroes and recreate characters that otherwise could not exist in the new continuity. Alternate timelines were also used, the most notable event being Armageddon 2001 in 1991. An Antimatter Universe existed as well, which had some "reversed" events in a similar way as the former Earth-Three. The Earth within this Universe was called "Earth 2". In addition, there was a Limbo, where some heroes and characters that could not be brought back to "existence" after the Crisis on Infinite Earths lived outside the Universe.An important rule in the new DC Universe was that there could only be one timeline, so any change caused by time travelers caused the destruction of their timelines of origin. Changes in the past were often "fixed" or have to comply with the present to prevent continuity errors.
Nevertheless, continuity errors appeared. The retold origin of Hawkman presented errors regarding the existence of the Golden Age version and the Silver Age version in the same continuity without explanation. The interaction of "possible timelines" also created continuity holes. This led to a new crisis to address the problem, titled Zero Hour: Crisis in Time!. The resulting universe had a slightly re-written story with no continuity errors even though it was acknowledged that reality-shattering events did happen. This Universe kept the concept of one universe, one timeline. Such a timeline was "mapped" from the beginning to the present at the back cover of Zero Hour #0 that also included certain key dates in the future.
The need to publish stories outside the strict DC Universe continuity led to the creation of certain DC imprints. Stories that set DC characters in different situations after the Crisis on Infinite Earths, were published by DC Comics under the Elseworlds imprint. None of these stories were ever intended to be included in the "real" continuity of the DC Universe.
Certain characters were reinvented in a mature context and were published under the Vertigo imprint. Most of the times, the characters depicted within the Vertigo imprint had no relation to the original DC Universe's versions, nor did the events have influence over the new Universe.
Later, under a special publishing deal with Milestone Media, DC Comics published a new series of comic books that told the stories of the heroes living in Dakota City, formed mostly by African American superheroes and other minorities. These characters lived in a universe separated from the DC Universe. The event Worlds Collide presented one of the first modern intercompany crossovers within the established continuity of the Universes instead of being "imaginary" and showed that there could be other universes or even multiverses outside the new DC Universe.
In a similar way to Worlds Collide, the crossover event DC vs. Marvel showed another in-continuity crossover with another reality completely separated from the DC Universe and that has a Multiverse of its own: the universe of Earth-616 in the Marvel Multiverse.
The universes were rarely referred to with specific names within the stories but were named in the "Real World" using the name of the editorial, imprint or even an element in particular. While in the comic books the concept of a "real" Multiverse was avoided, the Multiverse played an important role in cartoon series and live-action shows.
In summary, from 1986 to 1999, everything not happening in the "mainstream" continuity appearing in DC comics was either a non-canonical story or happened in a completely different and separate reality/universe/multiverse.
In 1999, the success of Elseworlds' Kingdom Come and other stories, led to the creation of the concept known as Hypertime in order to publish crossovers with those characters and the mainstream continuity. This structure gave "existence" to alternate timelines, stories in Elseworlds, appearances in other media and any other appearance of DC characters in the past. The main timeline or "Central Timeline" was like a river and all of the alternate stories were branches of it. Hypertime was similar to the former Multiverse as it allowed each and every reality ever published to co-exist and interact as most branches tend to return to the original stream. However, all realities existed within only one Universe.