This article originally appeared in the first edition of the Ultimate Music Guide published on May 19, 2017.

In the 2002 film Love Actually, Emma Thompson's character is seen listening to a song from Blue and telling her husband, played by Alan Rickman, that Joni Mitchell taught her "how to feel". Later, Thompson unwraps a Christmas present from Rickman, rips off the wrapping paper and finds a copy of Mitchell's 2000 album Both Sides Now. When Rickman tells her the gift is intended "to continue your emotional education", she realises that he is having an affair and escapes upstairs to the marital bedroom. There, she breaks down to the strains of the title track; Mitchell's heavily reworked, autumnal version of her classic song which she had first recorded, so springlike, on Clouds more than three decades earlier.

As a dramatic metaphor, the two songs brilliantly encapsulate the trajectory of the relationship in which Thompson's character is trapped. But the use of Mitchell's music in the film also holds up a mirror to her own journey from Blue to the Mitchell we encounter on Both Sides Now. At the time of its release, Both Sides Now felt like a valediction. Why else would one of the world's greatest songwriters put out an album of orchestral covers if she hadn't run out of things to say? The inclusion of world-weary versions of two of her own greatest songs seemed to confirm the sense of a creative life that had come full circle.

Not that Mitchell was admitting that she had reached a full stop. She told the Los Angeles Times she felt her "point of view is too realistic and reality is too bleak" to inflict another collection of her own songs upon a troubled world. "Even I wouldn't want to hear an album of that stuff right now," she reasoned. Yet she wasn't finished with articulating the pleasure and pain of love. She was simply going to do so with songs that came from a different perspective than that of a 57-year-old woman facing an uncertain future who, as she told The Times, was now "an old babe ... of an age when husbands run away with younger models".

Like others before and after, she hit upon the idea of reinvigorating her own muse by turning to the songs of a pre-rock'n'roll era; the music of Billie Holiday, Glenn Miller, Nat King Cole, Frank Sinatra, Duke Ellington, Lena Home, Judy Garland, Ella Fitzgerald and Broadway show tunes. These were the songs her parents had listened to when she was growing up and her father, Bill -who must have played many of them as the trumpeter in a Saskatchewan dance orchestra offered to help choose the tracks.

Mitchell had her own ideas, however. Not for her a random selection haphazardly plucked from the Great American Songbook, as Linda Ronstadt had done on a trio of l980s albums with Nelson Riddle, and Carly Simon had also done on the albums Torch and My Romance. She might have run out of new songs, but she still had artistic ambition, and Mitchell's covers album was to be graced with a concept-the songs would trace the arc of a love affair from beginning to end, constituting, as Larry Klein her co-producer, musical director and by-now ex-husband grandiloquently put it, "a programmatic suite documenting a relationship from initial flirtation through optimistic consummation, metamorphosing into disillusionment, ironic despair, and finally resolving in the philosophical overview of acceptance and the probability of the cycle repeating itself."

Klein's role in the project added another layer of emotional poignancy to the concept: he and Mitchell had spent 12 years together prior to their separation in 1993, by far the most enduring relationship of her much-documented romantic life. Still a key artistic collaborator, Klein described a rigorous process of song selection. "Being a writer herself, we were very careful to select songs that Joni could really feel were her own, that she sang with the same intensity as if she had written them," he explained. "If a song didn't hold up to her standards as a songwriter, she just didn't feel that she could immerse herself in it."

Warming to his theme, Klein cited Nietzsche in claiming that his ex-wife sang the compositions "as if she had written them in her own blood". That Mitchell approved the Nietzsche quotation for inclusion in the liner notes suggested she didn't disagree with this high-minded judgement of her interpretative abilities.

But then Mitchell had never regarded false modesty about her art as a virtue. Two years earlier, following the release of Taming The Tiger, she had given an interview to The Observer in which she'd likened her work to Mozart, Blake and Picasso, opined that her lyrics "have a lot of symbolic depth, like the Bible" and declared that her music was so original that it "needs its own genre name". It would have come as little surprise in this vaunting self-assessment if she had also announced herself to be a singer who should be listed with Callas, Holiday and Piaf among the great female voices of the century. She didn't; but Both Sides Now was certainly in part conceived as an opportunity to force the world to consider her artistry as a vocalist, the one aspect of her genius that she felt had received insufficient recognition over her recording career.

The first hint of the project came in the summer of 1998, before Taming The Tiger had reached the stores. At the Day In The Garden festival held on the original Woodstock site, she performed "Comes Love", composed by Sam H Stept, with lyrics by Lew Brown and Charles Tobias, for the 1939 Broadway musical Yoke/Boy.

By the time she joined Bob Dylan on a joint tour that autumn, she was confidently closing her set with "Comes Love". It was a bold statement of her desire to be taken seriously as a singer, for the song was famous for the versions recorded by Billie Holiday and Ella Fitzgerald in the '50s, yet she pulled it off with an undeniable panache. "The song made the most of her natural voice, its bright cutting vowels and its new depths, a gift of age. lt took her into a completely new context," remarked Ben Ratliff, reviewing her concert appearance at Madison Square Garden for the The New York Times.

When Dylan followed her on stage, he reportedly told her, "I'm going to sound like a hillbilly now." Did Mitchell's performance plant a seed that would lead to his own adventures into the Great American Songbook many years later? Only Dylan knows, but the favourable reception encouraged Mitchell in her ambition to make an entire album that showcased her talents as a singer rather than a songwriter.

As she selected the songs with Klein and began to arrange them into a narrative sequence, she realised that she could not have sung such songs with any degree of authentic sophistication in the "pure" voice of her younger, ingenue self, but that the vocal deterioration caused by age, experience and years of smoking had also brought with it a greater emotional depth and realism. In turn, this realisation led to the idea of incorporating some of her own songs into the narrative, on the basis that she could now bring an expression and nuance to them which she had only been able to hint at in the original versions.

It was a move fraught with danger, for artists re-recording their old hits is usually a fruitless exercise in disappointment. Yet there was one successful example that gave Mitchell encouragement. While she was planning the album that was to become Both Sides Now, Kris Kristofferson released The Austin Sessions, a set of wonderfully weather-beaten versions of his most famous material, including "Me And Bobby McGee", "Sunday Mornin' Comin' Down" and "Help Me Make It Through The Night" that sounded even better in his 63-year-old voice than when he'd first recorded them 30 years earlier. "I didn't really understand 'em when I wrote 'em," he said. "Now I'm beginning to get what they're about."

Mitchell endorsed the sentiment and decided to include "A Case Of You" to mark the halfway point in her song cycle, as love is beginning to turn bittersweet, and "Both Sides Now" (with the comma dropped from its original 1969 title) as its almost Zen-like philosophical conclusion.

Having selected the songs, in the summer of 1999, Mitchell, Klein and the American arranger and conductor Vince Mendoza flew to London to record the album at George Martin's Air Studios in Hampstead with a 90-piece orchestra, many of its members drawn from the London Symphony Orchestra. The orchestra was augmented by Herbie Hancock on piano, the jazz drummer Peter Erskine and bassist Chuck Berghofer, all of whom were already in London, while Wayne Shorter's sax and Mark Isham's trumpet would be overdubbed on returning to Los Angeles.

The album opens with a swooning, velvety arrangement of "You're My Thrill", first recorded by Al Bowlly in 1934 but later made famous by Billie Holiday and sung by Mitchell with an audacious, jazzy phrasing packed with an intoxicated, erotic charge. "At Last" was first heard in the 1941 film Sun Valley Serenade, played by Glenn Miller's orchestra, although the song is probably more famous as the title track of Etta James' 1960 debut album. There's barely a hint of James' R'n'B syncopations here, though, as Mitchell channels the opulent big band swing of the original.

"Comes Love" is another vocal tour de force for her dusky contralto, with some cool jazz soloing from Shorter and Isham, before the arc of romance begins its descent with two songs associated with Nat King Cole in "You've Changed" and the anguished "Answer Me, My Love", a No1 hit for Cole in 1954, both sung by Mitchell with smoky subtleties of tone and phrasing.

"A Case Of You", given a transformatively melancholic arrangement by Mendoza, offers the opportunity to measure just how much Mitchell's voice has altered. She can no longer hit the high notes and, as her biographer Brian Hinton noted, "The way she sings, which was once a whoop, is now a dying fall." Yet the loss is more than offset by the richer, deeper patina with which her voice has become coated. "Don't Go To Strangers" is lush and opulent, but you can hear her imploring soul aching. "Sometimes I'm Happy", a show tune from 1927 later covered by Holiday and Cole, is given a more jaunty arrangement, as Joni invests the words "when I hate you it's because I love you" with an emotional ambiguity that shows she knows her way around a lyric even when it's not her own.

The arrangement given Sinatra's "Don't Worry 'Bout Me" is smoochy, almost treacly, but Joni's vocal eschews oversize crooning and captures the song's resignation to perfection, while her unorthodox diction on "Stormy Weather" transforms an overfamiliar standard by distilling its melodic essence into something more hauntingly stark than the famous versions by Lena Horne and Judy Garland.

The mood lifts on Rodgers and Hart's witty "I Wish I Was In Love Again" before she ends with the magnificent reinvention of her own title track, the brooding orchestration reflecting her meditative reinterpretation of the lyric. To anyone who grew up on the original 1969 recording of "Both Sides, Now", to play the two versions back-to-back is akin to seeing your life flash by in the blink of a song.

Although the album was only a modest commercial success, it was a critical triumph, winning two Grammy awards for best pop vocal and best instrumental arrangement. Even those with little love for Mitchell as a songwriter were forced to acknowledge the potency of her "new" mature voice. Robert Christgau of The Village Voice, a longtime Mitchell detractor, begrudgingly admitted that she had proved herself "a major interpretive singer", while NME offered the backhanded compliment that "she sings like a singer, rather than warbling like a harpy, for the first time in her career".