Warner Brothers Infinite Batmen from Brave and the Bold
When you put them together, Marvel and DC have been publishing two continuous, multi-titled universes for more than 125 years. Trying to keep the rich histories of these books straight has been an uphill fight for the editors and the publishers, and it's a common problem in genre fiction. The instant an author has to account for more than two people in more than one book, she runs the risk of losing track of one of them for long enough to trigger a flood of enraged fan mail.
Writers and editors have created a handful of tools to try and deal with the array of logistical problems that pop up in these situations. Comic companies, writers and artists (and editors and writers in countless other genres) have used these tools as shorthand to provide in-story explanations for screw-ups, as creative fuel for interesting stories, or as ways to try and channel that fanboy letter writing verve into more constructive arguments, like "can Wonder Woman fight in heels because her calves are superstrong, or because she's constantly flying a half an inch off the ground so it only looks like she's walking?" Here are 7 of the most commonly used tropes for streamlining continuity problems:
7. Infinite Worlds
Art by Carmine Infantino
The first real attempt at unifying diverse comic universes was "The Flash of Two Worlds," a 1961 story from DC that had the first meeting of the Golden Age Flash, Jay Garrick (from WW2-era comics), and his Silver Age descendant, Barry Allen. Earlier DC stories had casually mentioned that the Golden Age stories existed as comic books to the new-ish iterations, but the two had never met before. "The Flash of Two Worlds" set up DC's parallel worlds structure, where the Golden Age heroes of the Justice Society were located on Earth-2, while the characters from the ongoing DCU were located on Earth-1. And thus was born the fictional shanghaiing of the Many Worlds theory.
This trope is used throughout fiction: sometimes to tell single stories (It's a Wonderful Life), sometimes for character exploration (everyone on Star Trek with a goatee except Sisko is visiting from their Earth-3. Sisko with a goatee is just being more awesome) and sometimes to add a sense of epic scale to stories that might not otherwise have it, likeFringe.
In the DC universe, it became shorthand for explaining away continuity errors and integrating characters purchased from other companies - the Charlton characters (Blue Beetle, the Question, Peacemaker, etc.) existed on one Earth, while the Fawcett characters (the Marvel family) existed on another. Elseworlds, DC's "What If" stories, also take place in discrete dimensions.
Marvel has distinct numbered dimensions going as high as Earth 989192, with many of them showing up through What If stories and a large number of them popping up through series designed to skip through the Marvel multiverse. Marvel also has the odd tendency of blocking off its lines - the X-books regularly interact with each other, but rarely with the rest of the MU; the same for Avengers or Spider-Man or the space comics. This makes the line feel like it's happening on multiple worlds. Sure, Wolverine is in almost every comic, but he shows up on a story by story basis, and for anyone other than Jason Aaron, trying to write out the beats in his daily schedule is usually futile.
Marvel also has the Nexus of All Realities, a hillbilly Dark Tower. In Stephen King's written universe, all fiction exists as separate levels of the tower, explaining why the Tower characters go through The Stand's Kansas and meet up with Pere Callahan from Salem's Lot to fight bad guys dressed like Dr. Doom wielding lightsabers and snitches from Harry Potter. In the Marvel U, the Nexus of All Realities is a gateway to each of their almost-a-million dimensions, as porous as a shitty gas mask and guarded by a handful of Florida swamp people and Machine Man.
6. Infinite's a Lot. What About "Several Worlds?"
Because there's apparently a cap to the amount of space available in the quantum multiverse, DC's current continuity status is a "let's not get too crazy" version of the infinite worlds trope, where the number of alternate universes is capped at 52. Someone at DC is obviously obsessed with playing cards.
DC has been sweating the symbolism of the number 52 since they couldn't come up with a name for their weekly series after Infinite Crisis. While deciding that there will be a finite amount of alternate worlds seems random, it isn't entirely unheard of. Planetary, Warren Ellis, John Cassaday and Laura Martin's book about superhero archaeologists in the Wildstorm Universe, said that the multiverse was a snowflake that existed in 196,883 dimensions and that theory was based on (possibly junk) popular math at the time.
It's also really hard to complain when a finite multiverse leads to stuff like this. Multiversity is one of the most compelling arguments for the existence of shared universes out there because of the absolute insanity that it encourages. Limits to the universe allow for symmetry in storytelling and for more structured symbolism, and in this case is pretty much soft toss for Grant Morrison.
5. Goddamn Time Travel
The Crisis on Infinite Earths collapsed DC's whole multiverse down to one Earth, and distilled 50 years of contradictory comics into one timeline. Shockingly, the condensation of a half century of stories into one set of stories led to some...issues...and the absence of a multiverse handcuffed the storytellers trying to figure it out. Reconciling the numerous different visions of characters that had been continuously published over the company's history after closing the door on past continuity was complicated - zany '50s Batman and hairy-chested '70s sex god Batman were almost diametric opposites. Although, now that I think about it, '50s Batman did wear a lot of animal prints...
In a lot of instances, DC approach to new continuity was "Screw it, we'll figure it out later, you guys do what you want." This lack of direction for their characters led to problems with characterization and plot details. So, to try and clear up any confusion leftover from the Crisis, DC had another giant crossover less than 10 years after the last one, using a freshly-insane Hal Jordan to rewrite the history of the universe from the beginning of time. That's right: time travel.
Time travel has worked in genre fiction exactly three times: Looper; the last episode of TNG; and the Age of Apocalypse. Looper because it was about a paradox, "All Good Things" because it had weepy Picard in a paradox, and the Age of Apocalypse because it was about a dark as hell paradox. Please note: all of these stories succeeded because they were about problems that arose from the act of time travel. None of these stories were trying to use Hal Jordan squeezing the baby universe to clear up why Donna Troy had a kid with Disco Stu.