There is a moment early in the 'Heiliger Dankgesang' movement of Beethoven's Quartet Op. 132 when, without ceremony, an alien, courtly trio is plonked down into the poised chorale underway. It is out of place, secular, overheard: we are left wondering how and when Beethoven will take us back to the chorale, which he duly does, courtesy of a few pivot chords and some shadowy harmonies. It is like those great moments in eighteenth- and nineteenth-century opera when the musical and narrative arguments onstage are interrupted by a banda off – the party Violetta throws in her Paris salon early in La Traviata, say, tuberculosis in the air. In Beethoven's imagination, though, it is played out in the most intimate of textures, that stitched together by a mere two violins, a viola, a cello.