CHARLES MINGUS WAS 56 and terminally ill with amyotrophic lateral sclerosis, also known as Lou Gehrig's disease (after the late New York baseball star), when a friend played him Don Juan's Reckless Daughter in early 1978. The bassist was intrigued by the cover - Mitchell in drag as a black man - and the extended improvising in Paprika Plains. Mingus - who had played with Dizzy Gillespie and Charlie Parker and written the landmark compositions Goodbye Pork Pie Hat and Haitian Fight Song but could no longer grip his instrument - contacted Mitchell, asking her to collaborate with him on a musical adaption of T.S. Eliot's Four Quartets.

She passed. Instead, the two worked together on a set of Mingus melodies, among his last, to which Mitch- ell wrote lyrics and vocal treatments.

"I felt this kind of sweet giddiness when I met him," Mitchell told Cameron Crowe. "Charles put on this one record, and just before he played it, he said, 'Now this song has five melodies going all at once. I said, Yeah, I bet you want me to write five different sets of words for each one of those melodies, right? And he grinned and said, 'Right.'"

Despite his worsening condition, Mingus attended a few initial sessions for the album that would bear his name, held at Electric Lady Studios in New York, where Mitchell was leading shifting casts of Hall Of Fame jazzmen: saxophonists Phil Woods and Gerry Mulligan; bassists Eddie Gomez and Stanley Clarke; guitarist John McLaughlin; drummer Tony Williams.

By the late summer of 1978, Mingus was in Mexico with his wife Sue, vainly pursuing alternative treatments, and Mitchell was in Los Angeles, re-recording most of the album with her own version of Weather Report - with Herbie Hancock on keyboards instead of Joe Zawinul. "The vibe I got," former Weather Report drummer Peter Erskine says, "was that she was being humoured a little" at the New York dates, "almost like, 'Get out of our way - we're the real jazz guys.' Joni came by a Weather Report rehearsal and that might have gotten some wheels spinning."

She had been working with a pianist, Jeremy Lubbock. But on the first day of recording in LA, during a pass at Goodbye Pork Pie Hat, Mingus's eulogy for saxophonist Lester Young, Pastorius came over to Erskine "almost in this conspiratorial way and goes, 'Who is this guy? I'm going to call Herbie.' And Herbie says, 'OK!'" Erskine says, still amazed. "Now Joni has to tell Jeremy, 'Thanks, but sorry, you have to go.'"

Hancock overdubbed a new piano part on Pork Pie Hat in one take. "Then we jumped into the next song, and the one after that." Wayne Shorter came in the next day. The band, with percussionist Don Alias, cut five of Mingus's six extended songs in those two days, including The Dry Cleaner From Des Moines, a tangled-brass, New Orleans-style march, and the jazz warrior's reverie A Chair In The Sky. Mitchell completed her memoir of Mingus, God Must Be A Boogie Man, two days after his death on January 5, 1979.

Mingus, ultimately, was not a jazz album; it was a meditation on dying, scored as a fight to leave in dignity and fulfilment. "To be honest, Mingus is not an easy album to listen to," Erskine says, an admission affirmed by many of its reviews. But, Erskine insists, "We knew this was something special. That was apparent. Even though we weren't in the 30th Street Studios in New York" - the legendary Columbia Records facility where Mingus made his 1959 classics Mingus Ah Um and Mingus Dynasty - "it was as if we were. Everybody knew this was something sacred."

MITCHELL TOOK Mingus on the road, leading a band with Pastorious, guitarist Pat Metheny, saxophonist Michael Brecker, and the a cappella vocal group The Persuasions. But it was the beginning of a long leaving.

Starting with Court And Spark, the singer had made five of her most compelling, uncompromised studio albums in seven years. After Mingus, she would not return to record until 1982's Wild Things Run Fast, and then issued new work between lengthening gaps over the next two decades, to diminishing sales. But at her commercial height, Mitchell found a renewing energy in jazz. The improvising camaraderie and harmonic liberation transformed her as a singer, record producer and impatiently progressive songwriter.

Guitarist Larry Carlton sums up Mitchell's mid- and late '70s this way: "Right artist, right songs, right players - and the world got to hear the goodness. I'm sure she was thankful for the success of Court And Spark, but Joni never tried to follow it up. She went on with the creative process - and the freedom."

"Pigeonholes all seem funny to me," Mitchell told Downbeat's Leonard Feather in 1979. "I feel like one of those lifer-education types that just keeps going for letters after their name - I want the full hyphen: folk- rock-country-jazz-classical... so finally when you get all the hyphens in, maybe they'll drop them all, and get down to just some American music."