Note: This article discusses specific plot details of the X-Men comic book storyline "Days Of Future Past" (among others), the basis for the X-Men: Days Of Future Past film. 

This weekend, comics fans—along with people who enjoy seeing Hugh Jackman shirtless, so everybody—will line up at the cinema to see X-Men: Days Of Future Past, the latest in an increasingly complicated series of superhuman melodrama summer blockbusters. Adding a new wrinkle to the story of Marvel's merry mutants, this movie involves time travel and alternate timelines, two sci-fi plot devices that can be confusing for even the most attentive fans. That's why I've put together an overview of the X-Men franchise's tangled parallel realities.

The basis of most alternate-timeline stories is the idea that every decision made in the present creates a different future. Take Back To The Future Part II. Old Man Biff swipes a sports almanac from the future and delivers it to himself in the 1950s, leading Marty to return to an alternate version of 1985 where Biff is rich and powerful. So Marty has to go back and stop it all from happening in order to restore his normal life. More recently, Community toyed with the same idea in "Remedial Chaos Theory," suggesting that the act of rolling a die creates six alternate timelines where everything is affected by the outcome of the roll.

X-Men: Days Of Future Past is based on the Uncanny X-Men arc of the same name. Written by Chris Claremont—whose tenure on X-Men included "God Loves, Man Kills" and "The Dark Phoenix Saga," the inspirations for the second and third X-Men films, respectively—the story revolves around young X-Man Kitty Pryde. Kitty has her consciousness sent back in time from a future war-torn America in order to prevent a political assassination that led to the hunting and slaying of mutants worldwide. An exciting and original story at the time, "Days Of Future Past" started a trend of X-Men stories about time travel and alternate realities, as seen below. In each of these stories, some major event led to a horrible dystopian future, and the X-Men have to change it back, with at least one hero remembering the events from one timeline to another.

(Click the graphic for a high-res version.)

These are not all of the known alternate realities in Marvel Comics—far from it—but these are the timelines where our heroes knowingly corrected history to return to the proper Marvel timeline (referred to as Earth-616). There are thousands of varied universes connected to Marvel so far, mostly thanks to "What if..?" comics that propose a single change to story arcs, side projects like the Millennial Visions books that allowed their creators to imagine whole new universes willy-nilly for the sake of a single short story each, and superhero teams like the Fantastic Four and Exiles who regularly hop between hundreds of different realities throughout their adventures.

Speaking of the Exiles, observant readers might have noticed there is one mutant who only appears once on those graphics: Blink. There are many characters who only show up in one or two timelines, but Blink is an important standout. Aside from appearing in the Days Of Future Past film, Blink was actually pulled from the "Age Of Apocalypse" timeline and led a team of similarly displaced heroes on adventures through hundreds—perhaps thousands—of similarly wild and wacky timelines. This one persistent version of Blink has easily visited more alternate realities than any other X-Men character, perhaps more than the rest of the X-Men combined.

Below is a brief rundown of a few popular alternate timelines that were never "corrected" and continue to exist—parallel to and independent of most Marvel comics. The diagram also catalogs alternate-reality versions of the most frequently appearing X-Men heroes and villains (along with a few Avengers for comparison's sake).

By and large, these universes are just an excuse to have some fun reimagining the course of events in the X-men's world—and to explore how the characters might get along (or not) if their lives played out differently. Sometimes, though, the alternate timelines provide engaging stories that are worth exploring deeply, regardless of their place in a grander continuity. Case in point: Bryan Singer has teased that the next film in the franchise, due for release in 2016, might be inspired by the "Age Of Apocalypse" storyline. It's not hard to imagine how the timeline-altering antics of Days Of Future Past might create a whole new dystopian world for the X-Men to defend. In any case, these charts should help novices get the bearings, as we only have two more years to catch up on our X-Men histories.