Continuity mistake: After Kramer walks into Jerry's apartment, he shuts the door. However, in another shot a couple seconds later, the door is inexplicably wide open. (00:01:45)
Continuity mistake: During a conversation with Kramer, Jerry is holding a drink in his left hand. Although he is never seen setting it down, the drink disappears in the next shot and he has his arms crossed. (00:02:10)
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