One of the most iconic Seinfeld episodes of all time is easily the one about the marble rye bread, and there's a unique story behind the reason it became the subject of an entire episode. While people around the world are familiar with - and eat - rye bread, a part of the Jewish cuisine in the United States includes the marble rye, a type of bread that consists of both traditional rye bread and pumpernickel.
Although a loaf of marble rye isn't considered a delicacy, it's still more highly regarded than sourdough or wheat bread, and because of its place in Jewish culture (at least in the United States), it became a key part of a George Costanza-focused episode of Seinfeld. In Seinfeld season 7, episode 11, "The Rye", George brings his parents to meet his fiance's parents, and his father chooses purchase marble rye on the way to dinner. But when the bread isn't served, he takes it back home, which sends the rest of the story spiraling out of the control.
From George wanting to return the marble rye to Susan's house, Kramer distracting Susan's parents by taking them around on a hansom cab ride, and Jerry trying to purchase an identical marble rye to replace the one George's father took home, it's easy to see how this particular episode became so memorable. Interestingly, one of the staff writers, Carol Leifer, drew upon experience from when her parents first met her ex-husband's parents, and her father waited for cake to be served before leaving. In fact, Seinfeld used actual dialogue from Leifer's parents for the episode. It's just that, in this case, instead of cake, it was marble rye.
All kinds of characters came and went on Seinfeld, but George's family was somewhat constant throughout, appearing sporadically and coming up with weird ways to make his life seem even kookier than he let on. For Leifer, "The Rye" episode was one of their first real opportunities to flesh out George's parents - Frank and Estelle Costanza, who were played by Jerry Stiller and Estelle Harris, respectively - but, of course, the one thing that they're perhaps most fondly remembered for is the tradition of Festivus.
Given that Seinfeld was such a popular show in the 1990s, it's not too surprising that marble rye ended up becoming quite famous after "The Rye" episode aired in 1996. Leifer mentioned in her Vanity Fair interview that delis started advertising that they served the famous marble rye after that, at least for a short time. So while it may not be as easy to find marble rye - the "famous" marble rye - nowadays, it's still something that can be closely tied to Seinfeld, even decades later.