Exactly what Mozart meant by "cassation" isn't quite clear, but this and its partner work, the Cassation in G, K. 63 (also scored for strings and pairs of oboes and horns), clearly belong to his sequence of serenades for orchestra. Its only unusual aspect is its key; Mozart generally preferred the festive D major for his serenades. Here, B flat lends itself first to a gracious, intimate march (Leopold Mozart, the father of the 13-year-old composer, preferred the French spelling of this movement and the two minuets). The ensuing Allegro molto, despite some rather generic thematic material, features lively writing for the strings. The Andante shifts to E flat, and again the strings play without oboe or horn reinforcement. The piece is elegant and lyrical, the muted violins and violas accompanied by pizzicato cellos; it's a work of unusual delicacy.

The first of the Minuets (or Menuets) employs a quick, assertive striding theme with hints of canonic writing that carry over into the much more lyrical trio section. The second Andante, for strings and oboes, moves to G minor, a key that always found Mozart at his most poignant and effective, even at this early date. Rather than the turbulence associated with this key in Mozart's late work, though, here we have gently flowing music, with noble variations that could almost be drawn from a Bach cantata.

The second Minuet is really more of a German ländler, but light-footed, with the central trio section scored for strings alone and featuring a teasing, rising melody. The final movement, a rondo, falls into several quite distinct sections. The initial, scampering Allegro in 2/4 gives way to a more lilting Andante, basically a siciliano in 6/8 meter; Mozart repeats these two sections with slight variants, and wraps everything up with a succinct repeat of the opening march.

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