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La Monte Young's magnum opus will have you entranced, bewitched and will change your sensory perceptions of the phenomenon we call music. It is a work for solo piano as you've never heard it before, and is simultaneously universal in its scope and intimate and enchanting in its effect, even as it opens the doors to new kinds of musical and psycho-acoustic experience.

Young, one of the fathers of American minimalism along with Riley, Glass and Reich, may have worked with and inspired everyone from Brian Eno to John Cale, once beaten Eric Dolphy to the second-alto saxophone position at the Los Angeles City College Dance Band, written pieces that require you to release butterflies into the auditorium and feed bales of hay to the piano (yes really; they're just some of the pieces that are part of his "Composition 1960" series), been among the first to realise and capitalise on the static time-flow latent within serialism – especially the crystalline pitch-structures of Webern – in a piece such as his hour-long Trio for Strings and founded the Dream House with his wife Marian Zazeela – a project/place/way of being in New York where time is based on a 27-hour clock – but even the ironically maximally long sentence required to sum all that up is only an upbeat to the perception-changing piece that will mean Young's music stretches into the cosmic infinite of music history: The Well-Tuned Piano.

Given that this piece has been one of the central pre-occupations of Young's life and work since the mid-60s, and given that you have the chance, here, to immerse yourself in it for more than six-and-a-half hours (or you can choose a shorter, five-hour performance, starting here), The Well-Tuned Piano is going to dominate this discussion. So the first thing is: don't panic. The length of this piece is deceiving, dynamic and ever-changing. What I mean by that is that The Well-Tuned Piano exists as a template for Young's continual improvisation and expansion (although the filmed performance, with lighting by Zazeela, from 1987 is the last time he played the piece in public), and that listening to the recording or watching the video collapses your sense of scale and time so six hours pass differently from how you might expect. In fact, time seems both to expand as well as contract.

The piece so takes over your brain that it seems as if it has always been playing, that no other music apart from this has ever existed; and yet, because of the way themes and ideas return throughout the performance, you're aware of a structure, you become conscious of musical anchors in the ocean of pianistic time that the piece creates.

Kyle Gann, The Well-Tuned Piano's most extraordinarily perceptive critic and advocate, sums up the delirious temporal mind-funk the piece creates: "In an 'eastern', timeless way [Young, like Terry Riley, was a student of Pandit Pran Nath], the work can be listened to as a static articulation of a set tuning, a continuous present in which concepts of before and after are irrelevant. In a 'western' way, it can also be heard as a unidirectional time-pattern of thematic development in which earlier melodies return altered and at different pitch levels. Perhaps it is more accurate to say, avoiding quotation marks and over-worked stereotypes, that the WTP [Gann's abbreviation] manifests with equal emphasis both poles of the psychic qualities immanent in all musical listening. Few works so satisfy the desires of both structuralist and ambient listeners."

All of that, though, is still in store for you – unless you've already started listening. Because the first and most significant thing that will strike your ears from the first second of The Well-Tuned Piano is the acidulous yet soft-focused beauty of the tuning system Young uses in the piece. The product of decades of research and of dissatisfaction with the artificial exigencies of equal temperament – that gigantic musical fudge that has arguably robbed classical music of much of its expressive power but to which our ears have unfortunately become almost completely accustomed – Young's tuning system is based on a fundamental E flat and a sequence of rational, ratio-based relationships for the other notes of the scale. Read Gann's distillation of what Young does and be as flabbergasted as I was to learn that Gann worked out much of this unbelievable Pythagorean subtlety using nothing more than his ears, a calculator and a synthesizer, blowing open once and for all the veil of secrecy about how Young created these ethereal yet earthy sounds: it's a musical hall of mirrors where, to obey the strictures of Young's tuning, the G sharp on the keyboard actually produces a note lower than the G; the C sharp and C similarly.)

The piece, in its 1987 incarnation, cycles through sections that Keith Potter, in his book Four Musical Minimalists, charts from The Opening Chord through The Romantic Chord, Orpheus and Eurydice in The Elysian Fields, and The Ending. But that's just the tip of the iceberg of the work's poetic titles: themes, motives and subsections of the piece rejoice in monikers such as The Theme of the Dawn of Eternal Time, Young Böse's Brontosaurus Boogie ("Böse" because Young plays a Bösendorfer piano, with its extension down a major sixth from the compass of a normal piano; instead of the bottom note being an A, the Böse goes down to C) or The Homage to Brahms Variation of the Theme of the Dawn of Eternal Time.

With Gann as your guide, you'll be able to follow the progress of the Well-Tuned Piano through its "clouds", themes and sequences. But what you'll also hear, if you pay enough psycho-acoustic attention, is a weird chorus of notes and timbres that don't seem to come from the piano, and which open an acoustic space far below the bottom reaches of the instrument. Gann heard a resonance of a truly fundamental E flat, the one that cycles just 18 times a minute, off the scale of what even Young's Böse can do, but the pitch from which all of the other notes in the piece derive as overtones; Keith Potter recalls hearing phantom French horns, saxophones and voices conjured from Young's keyboard. Even via YouTube, you'll encounter strange sonic phenomena as the clouds of resonance from Young's piano shimmer and hover in the air, taking on an almost physical reality in the course of the performance.

Young is now devoted to working with another of his groups, The Just Alap Raga Ensemble; whether The Well-Tuned Piano will be performed live again by him remains to be seen (his pupil/disciple Michael Harrison is the only other person ever to have done so). It is one of the great achievements of 20th-century music and to hear a performance must be one of the most remarkable and life-changing things you could ever hope to do as a listener. Meanwhile: enjoy your six hours and 40 minutes inside the world of The Well-Tuned Piano …

Five key links

The Well-Tuned Piano (1987)

The Well-Tuned Piano (1981)

Just Charles & Cello in the Romantic Chord

Pre-Tortoise Dream Music

B Flat Dorian Blues (The Theatre of Eternal Music)