In the 1950's, 1960's, and 1970's, the structure for sitcoms was one single plot line from the beginning to the end of the episode. An example would be a wife tries to keep the dent she put in the fender of the family car, a secret from her husband.
Starting in the 1990's, sitcom structure had changed. It no longer was one plot line for thirty minutes. It became one major plot line and one minor plot line. Let's take the comedy series Wings. The minor plot line of one episode was that the cab driver, Antonio, received a package from his late uncle. The box contained items indicating the uncle may have had an affair with a beautiful Italian movie star. The main plot line had Joe and Helen becoming friendly with their new neighbors who turn out to be psychotic.
But the radical change in structure occurred with the advent of the popular sitcom, Seinfeld and continued with Larry David's, Curb Your Enthusiasm. No longer did we have a major and minor plot. Those shows introduced its audience to three or four plot points that would all intersect by the end of the episode.
In one episode of Seinfeld, Jerry, George, Elaine and Kramer are invited up to the Hamptons to see the baby of a couple they're all friends with. This is the main plot line which everything else revolves around setting off the following events: George's girlfriend is seen topless by everyone embarrassing George. Then Jerry's girlfriend sees George naked after he swims in the pool causing George to worry about “shrinkage.” Elaine flirts with a doctor who thinks the couple's ugly baby is breathtaking. And finally, Kramer finds lobsters in the ocean which ultimately leads to his arrest.
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