In the last days of New York's Fillmore East, John Lennon and Yoko Ono joined Frank Zappa and the Mothers of Invention onstage after the Mothers' set. The four songs performed on June 6, 1971, were recorded and released on Lennon's 1972 album Some Time in New York City and later on Zappa's 1992 live LP Playground Psychotics.

The session had its genesis in a Lennon and Ono interview conducted by Village Voice writer Howard Smith on his WPLJ-FM show. At the end of the interview, Smith, who was off to talk to Zappa at the guitarist's hotel, asked Lennon if he would like to come along. Lennon, a fan of Zappa's music, said yes.

"A journalist in New York City woke me up – knocked on the door and is standing there with a tape recorder and goes, 'Frank, I'd like to introduce you to John Lennon,' you know, waiting for me to gasp and fall on the floor," Zappa recalled on his 1984 Interview Picture Disc. "And I said, 'Well, okay. Come on in.'

"And we sat around and talked, and I think the first thing he said to me was, 'You're not as ugly as I thought you would be.' So anyway, I thought he had a pretty good sense of humor so I invited him to come down and jam with us at the Fillmore East. We had already booked in a recording truck because we were making the Live at the Fillmore album at the time."

The Fillmore East audience, awaiting an encore by the Mothers, was surprised to see Lennon and Ono take the stage. The group opened with "Well (Baby Please Don't Go)," a 1958 tune by the Olympics. "This is a song I used to sing when I was at the Cavern in Liverpool," Lennon announced. "I haven't done it since."

"We went down there and I did an old Olympics number, the B-side of "Young Blood," Lennon told the BBC. "It was a 12-bar kind of thing I used to do at the Cavern. … It was pretty good with Zappa because he's pretty far out, as they say, so we blended quite well."

Ono's spacey vocals did blend well with the Mothers' freaked-out progressive rock. Three improvisational numbers followed: First was a take on "King Kong," a song from the Mothers' 1969 album Uncle Meat. Next was "Scumbag," in which the title is repeated over and over. (In case the audience didn't get the point, Ono was covered head-to-toe by a bag as she sang.) The jam wrapped with "Aaawk" (which was retitled "Au" on the Lennon album).

Three weeks later, the Fillmore East closed. When Some Time in New York City was released the following year, Zappa was surprised that "King Kong" was now called "Jamrag" – British slang for a sanitary napkin – and credited to Lennon and Ono.

"After they had sat in with us, an arrangement was made that we would both have access to the tapes," Zappa continued. "He wanted to release it with his mix, and I had the right to release it with my mix – so that's how that one section came about. The bad part is, there's a song that I wrote called 'King Kong' which we played that night, and I don't know whether it was Yoko's idea or John's idea, but they changed the name of the song to 'Jamrag,' gave themselves writing and publishing credit on it, stuck it on an album and never paid me. It was obviously not a jam session song – it's got a melody, it's got a bass line, it's obviously an organized song. Little bit disappointing."

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