Every generation or so comes a work that not only redefines the medium for which it's composed, but demands a new way of listening to it. Luigi Nono's string quartet Fragmente-Stille, first performed in 1980, is one such landmark piece. It was a stylistic watershed in the composer's own development, ushering in a final decade of increasingly rarefied, internalised music. But over and above that, it was a rethinking of the grammar of string-quartet writing. It's a masterpiece that had to be included in the Southbank Centre's Nono celebration, and the Arditti Quartet - who played the work to the composer, and have probably performed it more than any other group - duly obliged.

Fragmente-Stille is as much a study in the expressive power of silence as anything else. The etiolated gestures, blurred by microtones, often tremble on the edge of audibility, creating an environment in which the simplest gestures take on huge significance: a sforzando seems a seismic event, a chord that disintegrates into silence becomes profoundly tragic. This is a landscape of impermanence, a sequence of poetic fragments, in which something transcendent is always just around the corner.

The Ardittis inhabit this world with confidence, giving the silences and the music between them their full emotional weight. They framed the 35-minute piece with music from the Second Viennese School. First came Webern's microscopic Bagatelles Op 9, one of the few works in the repertory that could legitimately be seen as a precursor to Nono's. Afterwards there was Schoenberg's Second String Quartet, with soprano Claron McFadden in the last two movements, which seemed like a luxuriant hot bath after such austerity.