Even by Beethoven's standards, the Grosse Fuge represents a musical extreme. It is part of one of Beethoven's mythical late string quartets, works which were thought incomprehensible in their time and are still the peaks of the quartet repertoire.

The Grosse Fuge - literally Great Fugue - was originally the finale for the B flat major string quartet, Op 130. But the sheer size of the Grosse Fuge, which lasts for more than a quarter of an hour, and the incredible demands it places on the players, baffled audiences and critics at its premiere in 1826. Beethoven wrote a substitute finale, a much less technically and intellectually demanding piece. But his love for the fugue was never in doubt. It is that dedication to his most avant garde music that shines through this manuscript: Beethoven did not trust anyone else to arrange the four tortuous string parts for the keyboard.

Beethoven's sketches and manuscripts are notoriously complex, full of reworkings, rewritings, and cut-and-pastes, like looking into the white-hot crucible of musical creation, and this astonishing manuscript seems to be no different. It's as if Beethoven is wrestling with his own creation, trying to tame music he had already written for one medium and translate it into another. It all adds to the mystique of the music of the Grosse Fuge, still one of the most uncompromising pieces ever written, in which you can hear a composer at the height of his powers extending his imagination, the technique of his players, and the ears of his listeners into new realms of musical possibility.

· Tom Service is the Guardian's chief music critic