K. 66 B-flat major Allegro 4/4

The main material of the work consists of the opening motive comprising an embellishing tone and a leap, and the rhythmic motive of quarter notes and eighth notes appearing in the bass in measure 10. In the first half, the modulations to minor keys in measures 24 and 42, which omit the resolution of the preceding chord, create striking harmonic changes. From measure 75 in the second half, the repetition of a single motive, the dissonant sound of 7th or 9th chords, and the expansion of volume due to the increase in constituent notes of the bass chords gradually heighten the musical tension, forming a climax suitable for a middle section.