Sonata in B flat major K.570 (1789)

Mozart’s Sonata in B flat is deceptively innocent. In its simple two-part textures and its straightforward rhythms, it is scarcely more difficult to play than its immediate predecessor in C major K.545 which Mozart actually described as a sonata “for beginners.” Written in 1789, presumably as a teaching piece, it opens with a theme, a B flat arpeggio in octaves, even more modest than that which opens the Sonata in C major. And yet within a few bars the tonality has slipped into E flat major for an idea which is not the second subject but a chromatic diversion from the first. There is no true second subject: the opening arpeggio returns in the dominant and it is not until the closing theme of the exposition that any new melodic material is presented in that key. The diversionary material shares the development with the opening theme and, as a sure confirmation that Mozart did not intend it to be taken as a second subject, it is recapitulated in its original key of E flat.

The placid main theme of the Adagio is presented in such basic E flat major harmonies that (like the opening motif of Beethoven’s Sonata in E flat, “Les Adieux’) it evokes the sound of natural horns playng in thirds and fifths. It retains its stability throughout, acting as a kind of rondo theme round two episodes – one in a more agitated C minor, the other in a more expressive A flat major – until a variant of the second episode very subtly displaces it at the end.

The final Allegretto is another rondo and, in spite of its rather more racy style, similarly well behaved up to a point. The intervention, a less subtle one in this case, is an eccentric little passage which expresses its impatience with prolonged adherence to the tonic key by leading the harmonies astray in a percussive figuration of staccato repeated notes. Its influence persists almost to the end.