Plot hole: When George and Jerry are discussing The Rye bread scheme, George says Susan is working late that night, so she won't be around when he sneaks The Rye into the house. But when George reels The Rye in with the fishing pole, Susan is standing there with her parents, wearing her coat. When did she get home? She couldn't have entered through the front door, because Jerry was there tossing The Rye up to George. And since it's a New York City brownstone, there's no backdoor that's accessible. There's no logical explanation for her getting into the house to see George reeling in The Rye.
Continuity mistake: When Jerry and Elaine are talking in The Apartment, after George and Kramer go see the horse, the G+W Motorwerkes magnet disappears from the fridge.
Answer: Composer Jonathan Wolff used a synthesizer, although in seasons 7-9, a real bass is used in addition. Wolff also recorded himself making hundreds of mouth noises, pops, and slaps to add to the synthesized bass licks so that each episode has a different theme. The only real "back-story" is Jerry Seinfeld was having trouble coming up with a theme song and talked to a friend who happened to know Wolff. They wanted to avoid that cheesy late 80's sit-com theme song and Wolff came up with what we enjoy now. Jonathan Wolff has also talked about this further in interviews, recently Reed Dunela interviewed him, so for a fuller account of his story; check out "The Wolff of 116th street".
Bishop73