Friends and Seinfeld both existed in a shared universe. The NBC sitcoms shared many of the same aspects by focusing on an ensemble set of characters who lived in New York City. Although there was never a Friends and Seinfeld crossover event, certain details within the shows have proven they share the same TV universe.

Seinfeld first debuted in July 1989 and ran for nine seasons before coming to an end in 1998. The sitcom focused on the fictionalized version of Jerry Seinfeld as he lived in Manhattan's Upper West Side. The series also followed Jerry's best friend George (Jason Alexander), his ex-girlfriend Elaine (Julia Louis-Dreyfus), and his eccentric neighbor Kramer (Michael Richards). Friends overlapped with Seinfeld for five years, debuting in 1994 and airing for a decade before concluding in 2004. The series centered on a group of six friends - Monica (Courtney Cox), Ross (David Schwimmer), Rachel (Jennifer Aniston), Chandler (Matthew Perry), Joey (Matt LeBlanc, and Phoebe (Lisa Kudrow) - as they approached their thirties in the Big Apple.

Related: Friends: The Characters' Ages (& How They Get It Wrong)

The figure at the center of the shared universe between NBC sitcoms was Lisa Kudrow, but not in her portrayal as Phoebe. Kudrow additionally portrayed Ursula, the identical twin sister of Phoebe. Ursula first appeared in Friends during the season 1 episode, "The One With Two Parts." Ursula was just as odd as Phoebe, but unlike her twin, Ursula was selfish, brash, and spiteful. Before Ursula appeared on Friends, the character originated on the NBC sitcom, Mad About You. That series served as the connecting factor to the shared universe between Friends and Seinfeld.

How Mad About You Connected Friends & Seinfeld

The Cast of Mad About You

When Mad About You debuted in 1992, Ursula played the incompetent waitress at Paul and Jaime Buchman's (Paul Reiser and Helen Hunt, respectively) favorite restaurant, Riff's. In Ursula's Friends debut, Joey and Chandler went to Riff's and mistook Ursula for Phoebe where in the same episode, Jaime stopped in Central Perk and confused Phoebe for Ursula. This crossover proved that Friends and Mad About You existed in the same universe. Mad About You took it a step farther by crossing over with Seinfeld.

In Mad About You season 1, Jaime convinced Paul to sign away his former bachelor pad to the current tenant. That tenant turned out to be Kramer from Seinfeld. Paul even mentioned Jerry when asking about the man who lived across the hall. Later in Mad About You's run, Paul ran into Jerry while walking the streets of New York City. With Mad About You serving as the common factor between Friends and Seinfeld, it proved that all three sitcoms exist in the same NBC shared universe.

Even though all three sitcoms shared clever connections, there were a few discrepancies regarding the notion of a shared universe. One of the long-running gags centering on George in Seinfeld was his disapproval of his fiancée Susan's love for watching Mad About You. This joke was obviously meant to poke fun at a fellow NBC sitcom. In addition, Courtney Cox appeared as an entirely different character in Seinfeld season 5. The actress portrayed Meryl, Jerry's girlfriend in "The Wife". That said, the episode aired before Friends debuted its pilot so the plan for a shared universe may have still been in early stages of development.

Next: Friends & Seinfeld’s Rivalry Explained (& Who Really Won)