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Franchise movies have crossed over for over a hundred years, however despite the long and serialised nature of TV, they're still relatively uncommon besides spin-offs. Crossover episodes between shows indicate both shows take place in the same universe, and long running shows often have multiple over their course.

If you take all the crossover episodes of a TV show, and combine it with every crossover episodes from that show (et cetera), how many shows comprise the biggest universe? As "crossover" is quite vague on its own, I'm imposing these definitions on it:

  • The episode takes place at the location of one or both shows
  • The episode features at least one character from the second show, played by the same actor.
  • Movies don't count as episodes

For example, The CSI-Verse:

  1. CSI and CSI: Miami in Cross-Jurisdictions
  2. CSI: Miami and CSI: NY in Manhattan Manhunt
  3. CSI and CSI: New York in Hammer Down
  4. CSI and Without a Trace in Who and What

There are additional crossover episodes within the shows, but only unique shows count here, so this is 4. Though soon it will be 5 as the CSI episode Kitty has been commissioned for a full series.

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Do time travels count? I'm thinking of Star Trek - the different Series are connected via time travels (But maybe it is in conflict with the 'movies don't count'-rule). – knut Jun 2 '14 at 18:51
@knut Yep they count – Crow T Robot Jun 2 '14 at 18:56
I just don't like a question that can't have a definitive answer. It seems more like a discussion if that's the case. – Kevin Howell Jun 2 '14 at 19:23
@knut: Does Star Trek count because of time travels? The different series have a lot more connections that are not time travels (even when discounting the very sparse character cameos), and moreover, it's not even true - TNG, DS9 and VOY all play within the course of a few decades and at least TNG + DS9 as well as DS9 + VOY are not linked pairwise by time travels at all. In the realm of fantasy/scifi, I suggest "The episode features a fictional civilization/fictional organization from the second show." as an additional definition, though the word "crossover" seems like an understatement. – O. R. Mapper Jun 3 '14 at 7:01
@KevinHowell Ok, I tended to ignore that last sentence, since I didn't like it so much either, probably for the same reasons you did. – Napoleon Wilson Jun 3 '14 at 17:02
up vote 45 down vote accepted

According to The Tommy Westphall Universe Hypothesis, nearly all of American television takes place in the mind of an autistic child. The theory first appeared in 2002, when writer Dwayne McDuffie wrote Six Degrees of St. Elsewhere for the Slush Factory.

For those of you don’t know, St. Elsewhere was a slick, well written and acted drama series about the doctors, administrators and patients of the fictional Boston hospital, St. Eligius (nicknamed St. Elsewhere by the staff). After a long, award-winning run, in the very last moments of the show's final episode, it was revealed that all of the events of the show were merely the prolonged daydream of an autistic child. None of it "really" happened. Whether you like this final twist (for what it’s worth, I didn’t), it’s a legitimate ending to a self-contained show. But if St. Elsewhere played by the rules of comics, either they wouldn’t have been allowed to do it, or they would have precipitated a crisis in TV Land far bloodier than DC Comics' Crisis On Infinite Earths. Why? Because crossover-wise, St. Elsewhere is the Kevin Bacon of TV shows.

Stay with me now, this is complicated but kind of fun.

Characters from St. Elsewhere have appeared on Homicide, which means that show is part of the autistic child’s daydream and likewise doesn’t exist. It gets worse. The omnipresent Detective John Munch from Homicide has appeared on X-Files, Law & Order and Law & Order: SVU. Law & Order characters have appeared on Law & Order: Criminal Intent. X-Files characters have appeared on The Lone Gunmen and Millennium. Characters from Chicago Hope have appeared on Homicide. Characters from Picket Fences have appeared on Chicago Hope. All those shows are gone (if you count cartoons, which makes this game much too easy, the X-Files characters have appeared on The Simpsons. The Critic has also appeared on The Simpsons. Dead).

Characters from Picket Fences have appeared on Ally McBeal. Ally McBeal has appeared on The Practice. Characters from The Practice have appeared on Boston Public. Autistic daydreams, every one.

But that's not all. St Elsewhere characters have appeared on Cheers, so Fraiser doesn’t exist. Neither do Wings, Caroline In The City or The Tortellis but who cares? Well, maybe you do, because Caroline In The City once crossed over with Friends, which crossed over with Mad About You, which crossed over with Seinfeld and The Dick Van Dyke show. None of them happened in our new, shared continuity.

St. Elsewhere also shared characters with The White Shadow and It’s Gary Shandling’s Show. Garry Shandling crossed over with The Andy Griffith show (no, really!). So Gomer Pyle, Mayberry RFD, and Make Room for Daddy/The Danny Thomas Show are gone. Make Room For Daddy takes out I love Lucy.

And there’s more, St. Elsewhere also shares continuity with M*A*S*H, so Aftermash and Trapper John MD are out of there.

Now here’s a good one, St. Elsewhere shared a patient with The Bob Newhart Show, so the Bob Newhart Show is part of the grand daydream. The Bob Newhart Show crossed over with Murphy Brown, which in turn links to, among many others: Julia, The Nanny, Everybody Loves Raymond and I Dream of Jeannie! Meanwhile, the series Newhart was revealed to be a nightmare had by Bob Newhart’s character on the Bob Newhart Show. Newhart crossed over with Coach, which connects it to Grace Under Fire, Ellen, and Drew Carey. Drew Carey takes out Home Improvement and NYPD Blue.

All of these shows (and many more that I left out or missed) are daydreams of St. Elsewhere’s autistic kid.

That's forty-seven shows, by my count, that exist in this shared universe. You can also add Boston Legal, which shares many characters with The Practice, and both The Single Guy and Hope and Gloria for their crossovers with Friends to bring the count to an even fifty. Wait, no. Joey makes fifty-one. (We can keep adding to the list, since many shows that came out after 2002 cross over with shows already mentioned.)

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I would like to see a graph depicting all of these relationships... – Michael Jun 2 '14 at 21:59
In addition to the objections listed to this hypothesis, it is also interesting to note that in some series, a cross-over is done with a show that is (either earlier or later) referenced as being fictional. For instance, if on the show two characters talk about some detail of Joey on "Friends" the night before and then the following week they meet the actual Joey Tribbiani, which is it? Is he real (on the show) or not? – Michael Jun 2 '14 at 22:07
@Michael Good question. That reminds me, Joey played a character on Days of Our Lives on Friends. I don't know if that links in Days or not. – Bill the Lizard Jun 2 '14 at 22:29
According to The Master List, Tommy Westphall can be linked to nearly 400 shows. – Plutor Jun 3 '14 at 1:17
@Michael The site links to a graphic that you can zoom and pan on. Seems pretty readable, although it's not very pretty. – Kris Harper Jun 3 '14 at 14:01

Detective John Munch, played by the same actor, Richard Belzer, has appeared in at least a single episode of the following distinct television series, though in some cases, multiple episodes:

"Law & Order: Special Victims Unit"
"30 Rock"
"The Wire"
"Arrested Development"
"Law & Order: Trial by Jury"
"The Beat"
"Law & Order"
"Homicide: Life on the Street"
"The X-Files"

Using him as the singular connection point, nine different television universes can be seen to be paired. The character John Munch also appeared on Sesame Street, however, not played by the same actor.

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I don't know the Arrested Development episode with him (maybe from the 4th Netflix-season?), but judging from the show's nature, that might be hard on the edge of parody, but maybe that edge is hard to define anyway. – Napoleon Wilson Jun 2 '14 at 17:41
The episode is from season 3 - if you watch the clips, you find George Michael planning a birthday party for Maeby, from her address book. Seeing as she's a fake TV producer in that season, it probably has Munch as a party attendee. – wbogacz Jun 2 '14 at 17:56

There were the Beverly Hillbillies/Green Acres/Petticoat Junction crossovers in the 1960s.

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What about the Happy Days Universe?

  • Happy Days

  • Joanie Loves Chachi

  • Laverne and Shirley

  • Mork and Mindy

  • Sabrina the Teenage Witch

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Sabrina the Teenage Witch? How is that linked? – Jamie Taylor Jul 23 '14 at 7:13
Sabrina actually met Potsie in an episode of Sabrina the Teenage Witch – Kevin Jul 24 '14 at 18:21

Buffy and Angel? Quite a few episodes there.

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The problem is, though, that the question seems to be more after how many shows and not episodes, which in your case would only be 2 and thus lower than any of the already existing answers. – Napoleon Wilson Jun 3 '14 at 9:29
If we're counting episodes, then Doctor Who/Doctor Who Original Series/Torchwood/Adventures of K9/Sarah Jane Chronicles beats Buffyverse flat. – DJClayworth Jun 3 '14 at 16:44
It seems I misread the question, my apologies. – Jags Jun 3 '14 at 21:43

If you’re looking for a television crossover universe with the greatest number of episodes, you should probably be looking at daytime television soap operates which have been on the air for decades and have aired thousands of episodes. For example As the World Turns (13,858 episodes) spun off Somerset (1710 episodes) and Our Private World (38 episodes). It has crossovers with characters from Another World (8891 episodes) and The Young and the Restless (10,692 episodes and counting) which has in turn crossovered with The Bold and the Beautiful (7103 episodes and counting). By my count that’s well over 33,000 episodes among these series – well more than all of the Star Trek series, Doctor Who and related spinoffs or the Tommy Westphal-connected series.

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As repeatedly explained, he does not look for the most episodes. – Napoleon Wilson Jun 20 '15 at 16:19

Chicago Fire and Chicago PD Crossover and Law & Order Arrow and The Flash The Vampire Dairies and The Originals

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This answer doesn't give a bigger crossover than the already accepted answer. – invalid_id Nov 5 '14 at 11:37

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