When Schubert began writing his symphony in B minor in the autumn of 1822, the 25-year-old Viennese composer was charting new musical terrain. His first six symphonies - he cut his teeth on the genre as a teenager in a series of miraculously joyful pieces from 1813 to 1818 - were four years and a compositional epoch ago; two subsequent attempts at symphonies wouldn't get beyond the sketch stage. The most recent masterpieces in the genre were Beethoven's 7th and 8th, premiered in 1813 and 1814 in Vienna. By 1822, Schubert was ready to attempt in the symphony what he already done in his songs and had started to glimpse in his piano sonatas and chamber music. Instead of trying to take Beethoven on at his own game of dynamism, dialectic, and confrontation, Schubert found in the music he completed for this B minor symphony a way of shaping time and tonality that no other symphonic composer up to this point had managed. In terms of the history of the symphony, this music is unprecedented. To borrow Nikolaus Harnoncourt's phrase (who was originally talking about the draft of the finale of Bruckner's unfinished Ninth Symphony), what Schubert finished of this B minor symphony has all the strangeness, surprise, and shock of a "stone from the moon".

What we know today as Schubert's Unfinished Symphony is the two movements: an Allegro moderato and Andante con moto. And while there are many musical reasons for its extraordinary power, there may be some biographical factors, too. The syphilis that would kill him six years later had its first serious effects on Schubert's health in 1822, and while it's an affront to his achievement in this symphony (or, say, the A minor piano sonata written at the start of 1823, whose expressive world and musical rawness are, if anything, even bleaker) to limit the music to an interpretation that ties it too closely to the biography, there's a fearlessness and directness about this symphony that may come from Schubert's experience of a world of darkness and pain he had not previously encountered.

The music sounds its strangeness from the very beginning. Instead of the self-confident theme, statement, or energy that classical and early romantic symphonies should start with, this symphony opens with a ghost, with music that sounds like a revenant of a dream. A pianissimo shadow in the cellos and basses functions as an eight-bar introduction to another musical spectre, the first theme proper of the symphony, an embodiment of melancholy in the oboe and clarinet over a nervous shimmer of semiquavers in the strings. Schubert's orchestration signals a different spiritual dimension to this music as well: trombones, last used in a major symphony to triumphant effect in Beethoven's 5th, connote something different here. Used throughout both movements, they hark back to their earlier symbolism of the numinous and the uncanny (for example as in Mozart's Don Giovanni, in which they are associated with the Commendatore's ghost).

In place of a highly wrought transition to the major-key second theme, there's a musical cross-fade after the orchestra's climactic B minor chord, a harmonic sleight of hand in a few seconds of music as the horns and bassoons magic the music to G major. Schubert unveils another pianissimo theme in the cellos and then violins whose apparent major-key serenity, over a gently syncopated accompaniment - like a supernatural accordion - is really a tonal and emotional illusion. In mid-flow, just before you think the music's going to comfortably cadence again, Schubert pulls the rug out from under your ears - so to speak. There's a breathtaking pause, and then a plunge into a scalding minor-key fortissimo chord. The rest of the first section stabilises the music's trajectory into G major. But that tranquility doesn't last for long, as Schubert composes another revelatory few bars that lead back into the spectral opening - if the conductor observes Schubert's repeat sign - as he or she should do - or on into the works' central section.

This central section confronts the ghost of the very start of the symphony head on. This is the Unfinished Symphony's chilling heart of darkness: the theme in the cellos and basses is brought from out of the shadows to be revealed with a devastating glare. Apart from some haunting reminiscences of the accompaniment of the serene second theme - now sounding all the more disturbing in this precarious context - the whole of the middle of the movement is based on that opening music. Schubert conjures some extraordinary textures: the tremolo and slow chromatic ascent in the low strings that creates heartbreaking dissonance; the repetition of a sequence of ever-more intense phrases that builds up to a full, fortissimo encounter with the symphony's musical apparition, which in turn catalyses music of menacing energy and contrapuntal ferocity - before the movement returns to the oboe and clarinet theme we heard earlier. The reprise of both minor and major-key themes finds new strangenesses in the way Schubert subtly alters what we've heard, as if the music were infected by the darkness we have experienced. The end of the movement is no less remarkable: that ghostly theme returns, but Schubert manages to wrest the music towards a B minor resolution instead of another existential exploration of its musical and emotional possibilities.

The second movement, in E major, is also in three beats to the bar, and many conductors take a similar if not identical tempo in both movements, which amplifies the strange sense of unity across both pieces. There are specific thematic and gestural connections between them (compare the first cadence in the Andante in the bassoons with music you've recently heard at the end of the first movement), and on a larger scale, the movements are almost like negative images of each other: you've got a minor key first theme in the Allegro, but major-key opening melody in the Andante; a major key second theme in the first movement, and a minor key second melody in the second (keeping up?... good!). What's more, the second movement's minor-key theme floats above exactly the same gently throbbing rhythmic accompaniment that the first movement's second theme does - and the calm of the Andante's opening melody is yet another illusion, as it melts into weird keys and chromaticisms along the way. And in a piece full of sleights of ear, the slow movement has some of the symphony's most discombobulating transitions. Marked with three ppps to emphasise the weirdness of what's going on, the first violins twice tease the music into new harmonic realms with just five unaccompanied notes - a stroke of uniquely Schubertian genius - just after you think you've got back to the right key; once, into A flat major, and then into what's really F-flat major but is actually, enharmonically speaking, the home key of E major, just before the end of the movement… Told you this was illusive music!

Why didn't Schubert write more of the symphony, apart from 20 orchestrated bars of a fragment of the scherzo? (You can hear the fragment in Jonathan Nott's recording; Charles Mackerras gives you Brian Newbould's completion of the whole movement and a speculative finale, the Entr'acte that Schubert wrote for the play Rosamunde.) The reasons can only be guesswork: whether they're psychological, connected to the period of illness he went through; musical, in the sense of not feeling he could compose another two movements that would satisfactorily complement the new symphonic dramaturgy of the two completed ones; or simply practical, that having put the piece to one side, he wanted to get on with new projects rather than return to older music? Whatever the reason, it all conspired to mean that the Unfinished Symphony wasn't premiered until 1865 in Vienna - when it would still have sounded ahead of its time. Schubert's C major symphony, known as the Great, which he would complete in 1826, takes a different, more extrovert approach to the symphonic project; only Bruckner could be said to follow or continue the Unfinished's true legacy. "Unfinished" it may be in a strictly four-movement structural sense, but this B minor symphony is a complete, essential, and mysterious symphonic experience.

Five key recordings



Chamber Orchestra of Europe/Claudio Abbado: Abbado's Unfinished is miraculously satisfying; some revelatory playing from the COE.

Berlin Philharmonic/Günter Wand: Wand's live performance breathes a lifetime of experience of this repertoire.

Bamberg Symphony Orchestra/Jonathan Nott: Nott's is a emotionally and musically extreme - and in the first movement, daringly slow - view of this piece.

Vienna Philharmonic Orchestra/Carlos Kleiber: Kleiber's recording sings in a single symphonic arc from beginning to end.

Orchestra of the Age of Enlightenment/Charles Mackerras: on period instruments, Mackerras's version also lets you hear speculative completions and realisations of the B minor's symphony's scherzo and finale.