It was revealed in a MaY episode that Paul Buchman subleted his old apartment to Kramer (of Seinfeld), Jamie Buchman (of Mad About You) once appeared on Friends -showing up in Central Perk, Caroline (of Caroline in the City) once showed up on Friends and Ross (from Friends) showed up on "The Single Guy." NBC once did a "Black Out Thursday" where 3 of the night's shows (Mad About You, Friends, and The Single Guy) all suffered from the same blackout caused in MaY. (Seinfeld declined to participate in the TV event.)
All shows too place "in the same universe" though there's contradictions, Seinfeld for example made reference to "Mad About You." Though it's possilbe "a Mad About You" exsisted in Seinfeld's universe just not necessairly the exact same one *we* saw in the real world. (I believe The Single Guy also made reference to the show "Friends" existing in their universe. Again, it's like a show called "Friends" exsists but not necessairly the same Friends we all saw in the real world.)
Man, TV and sitcoms were fun in the 90s.
If I'm not mistaken, I believe there was an episode of "Mad About You" where Carl Reiner made an appearance as Alan Brady--putting the whole thing squarely in the universe of "The Dick Van Dyke Show".
Also there used to be crossovers all the time between "The Beverly Hillbillies", "Petticoat Junction" and "Green Acres". This, in spite of the fact that the same actress, Bea Benaderate, played both Cousin Peal Bodine on "Hillbillies" and the owner of the Shady Rest, Kate Bradley, on Petticoat Junction. Later, after Bea Benaderate became ill, June Lockhart took over the running of the Shady Rest, giving it a connection to "Lost in Space" (though Lockhart was NOT playing Maureen Robinson).