Crossovers are a common occurrence in fiction but for an obsessive fan like myself, they present a problem. How do I know if two different series take place in the same universe or not? Many crossovers are just what-if fight scenarios or cameo appearances that are not meant to be taken seriously. http://en.wikipedia.org/wiki/List_of_crossovers_in_fiction Some of the time I know instinctively that a crossover is ‘true’, (for lack of a better word) if the two series share similar themes. For example, Alien and Predator are both about alien monsters that kill people. A Nightmare on Elm Street and Friday the 13th are both about supernatural serial killers. Batman and Superman have had numerous crossovers with Aliens and Predators, the DC universe has encountered the Mortal Kombat universe, Picard has met the X-Men and Star Wars characters appear in the latest Soulcalibur game. But it's obvious that these crossovers are wrong because the different styles clash enormously. I mean can you imagine Superman fighting Aliens? It’s ridiculous. Other times, it’s not so clear. We all know that the Marvel superheroes belong in the same universe. The DC superheroes also belong together in a different universe. But DC and Marvel have had numerous crossovers and yet the crossovers don't imply a shared universe even though both series have a similar theme of a world overpopulated with superheroes. I've decided that crossovers that happen in a comic book should never be taken seriously because there are so many 'VS' comics that pit characters from widely different realities against each other in very implausible situations. Video game crossovers shouldn't be taken seriously either, considering such series as Marvel vs Capcom, Super Smash Bros and Kingdom Hearts, all of which blend dozens of completely incompatible realities in a huge mess. http://en.wikipedia.org/wiki/Intercompany_crossover Besides, not everyone plays video games, especially video games that are limited to one or two consoles. Not everyone collects comics either. A lot more people watch TV and read novels. At least that’s what I think. Am I wrong? So I decided that if a crossover happens in a film it is ‘true’. At least, that’s what I thought until I heard of The X-Files episode Unusual Suspects, which featured Detective John Munch. Munch was a main character on the show Homicide: Life on the Street, later became a main character on Law & Order SVU and appeared occasionally on nearly all the Law & Order shows as well as a number of completely unrelated shows. http://en.wikipedia.org/wiki/John_Munch This would suggest a universe with at least 12 TV series, many of which have nothing to do with each other, and more episodes than Star Trek. The X-Files series and Law & Order series have radically different styles. One is a sci-fi show, the other is a police procedural. There’s no way all these different series belong in the same universe. Especially because in the Homicide episode ‘Partners’, detective Munch remarks "It's 9 o'clock. They're probably home watching the X-Files." Therefore, there needs to be at least 2 film crossovers (or 2 novel crossovers) between 2 series for them to exist in the same universe. But that means the Nightmare on Elm Street series and Friday 13th series are separate because there was only one crossover movie. It also means that The X-Files and Millennium are separate because there was only one crossover episode and that clearly isn’t right because the X-Files/Millennium crossover was a critical resolution of Millennium’s storyline. I’m aware that the character Jose Chung appears in one episode in both series but does that even count as a crossover? So how does one decide whether or not two or more series take place in the same reality? Any more ideas? P.S. Please don’t mention the Tommy Westphall universe. It’s an extremely ridiculous idea and I’m concerned that it will lead this discussion in the wrong direction. Besides, only the first season of St. Elsewhere has been released on DVD, and if a show doesn’t exist on DVD, it doesn’t exist.