A guitarist whose playing is always full of surprise and discovery, Joni Mitchell boasts a style that seems to stand outside the accepted conventions of folk, pop and rock music. Listen to her songs and you will hear chords so unconventional that you think you've never heard them before.

Specialist guitar magazines devote entire articles to analysing her tunings and unusual harmonic structures. At times her guitar doesn't even sound like a guitar at all. As the American magazine Acoustic Guitar once put it, "The way she tunes her strings to the way she strokes them with her right hand, is utterly off the chart of how most of us approach the guitar... 0the treble strings become a cool-jazz horn section; the bass snaps out syncopations like a snare drum; the notes ring out in clusters that simply don't come out of a normal six-string."

Except that in Mitchell's hands, somehow they do.

Mitchell was a late starter on the instrument. As a child she had piano lessons and didn't buy her first guitar until 1964, when she was 20. At first she played in standard folk tunings, learning from Pete Seeger's How To Play Folk-Style Guitar. However, childhood polio had left her with a weak left hand and she began to compensate by experimenting with open tunings copied from old blues records, initially in G and D. As her songwriting developed, she grew more adventurous, creating her own unique tunings, several of which she cannot now remember.

The result is that even among her earliest compositions, only "Tin Angel" and "Urge For Going" are in standard tuning. Astonishingly, we have to wait more than 30 years for her next, "Harlem In Havana" from the 1998 album Taming The Tiger. Even some of her most straightforward-sounding folk-styled songs, such as "Both Sides Now and "Chelsea Morning", were actually composed and played in open tunings. As a result Mitchell could create the most complex chords full of unusual consonances, dissonances and drones by the simplest fingerings, and the discovery became the foundation of her music.

Joni herself was once asked to describe her guitar style. "It's closer to Debussy and to classical composition, and it has its own harmonic movement which doesn't belong to any camp," she answered. "It's not jazz, like people think. It has in common with jazz that the harmony is very wide, but there are laws to jazz chordal movement, and this is outside those laws for the most part."

By her own calculation, Mitchell has used 51 tunings in her songs over the years, each offering its own universe of different possibilities. She compares this continual changing to sitting down at a typewriter on which the letters are rearranged each day. "If you're only working off what you know, then you can't grow. It's only through error that discovery is made, and in order to discover you have to set up some sort random element, a strange attractor, using contemporary physics terms. You're constantly pulling the rug out from under yourself, so you don't get a chance to settle into any kind of formula:

It's this that marks her out as such an extraordinary and unique guitarist, and it's fascinating to trace how her use of tunings and the freedom it confers has allowed her to grow ever more audacious over the course of her long career.

Mitchell's first five albums were essentially solo works, built around her guitar, dulcimer, piano and voice. But even here there's substantial growth. Her debut album 1968 featured some delicate fingerpicking on top of her unusual chords, but by Blue, her fourth release in 1971, she was coaxing extraordinarily rhythmic and percussive sounds out of the guitar with her right hand, perhaps because many of the albums songs had been composed on a dulcimer. By 1974's Court And Spark she was working with a full band. Many of her favoured musicians had jazz training and by Hejira, in 1976, she was working with the bassist Jaco Pastorius. His style revolutionised electric bass playing, but it was Mitchell's unique use of chords and sense of harmonics that enabled him to do so, while his singular approach in turn influenced Mitchell's guitar-playing.

When she came to record Chalk Mark In A Rain Storm in 1988, Mitchell had grown so bold she was recording 16 and even 24 guitars on some tracks. Never one to shy away from technology, and in the mid-'90s, she again showed her willingness to experiment when she began playing a Stratocaster-style guitar with a Roland VG-8. A 'virtual guitar', it allows the strings physically to stay in the same tuning but alters the tones coming out of the speakers. Not bad going for a woman routinely dismissed a flaxen-haired folkie.

Indeed, Mitchell's image is about as far removed from that of the traditional guitar hero as it's possible to be. It surely also won't have escaped your notice that she is the only female to appear in this list. Tellingly, when Mitchell began working with a full band in the early-'70s, she had problems with male guitarists who'd try to stamp their own personality over her music. "I'd end up trying to tell them how to play," she recalled many years later. "And they'd say, 'Isn't it cute? She's telling me how to play my axe, and I've played with James Brown.' It was difficult as a female to guide males into playing what I wanted."

And while this is not the place to rehearse the familiar rant about the on-going sexism of rock'n'roll, it's worth pointing out that Mitchell's presence here is precisely because she is so different and plays in her own unique style. In other words, she's not competing with the boys because none of them play remotely like her.

Did you Know?

Joni was scheduled to appear at Woodstock, but her manager made her turn back en route as "the traffic was too bad". She wrote "Woodstock" to make up for it.