Franchise movies have crossed over for over a hundred yearsover 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.
- Parodies of a show featuring a cameo by an original cast member do not count (e.g. Saturday Night Live and Breaking Bad)
- Movies don't count as episodes
For example, The CSI-Verse:
- CSI and CSI: Miami in Cross-Jurisdictions
- CSI: Miami and CSI: NY in Manhattan Manhunt
- CSI and CSI: New York in Hammer Down
- CSI and Without a Trace in Who and What