Allegro Assai
In keeping with the Affekt associated with the key of C-major, and in contrast with the profound emotional landscape of the second partita, the opening movement of this sonata portrays simplicity and purity: The work opens with a single middle C and the simple motive Do-Re-Do.

Interestingly, the keyboard version of this Adagio is anything but simple. Of all the movements from the Sonatas and Partitas that were transcribed in Bach's time, this one's keyboard counterpart is the most developed, with intense harmonic, melodic, and rhythmic elaborations throughout.

Also in contrast to the simplicity of the Adagio, is the Fuga that follows it. Written in four-part invertible counterpoint, it is arguably the most complex movement in the collection.

There are several clues that point to a lost organ version of this Fuga, and quite possibly the rest of the sonata as well. First, there are keyboard arrangements of both other fugues (BWV 539 and BWV 964); and this one would lend itself very well to added counterpoint and fuller harmonies, which a brilliant mind like Bach’s would find hard to resist. In fact, records show that Bach improvised a fugue on the organ as part of an audition in 1720, with a similar subject. Also, as it does in the violin version, the keyboard version of the Adagio closes with a final cadence in the key of C, which is followed by a two-bar coda that takes the listener to a half-cadence on the dominant, heralding the fugue that would inevitably follow. ​

Following the Fuga, Bach returns to the Affekt of simplicity and purity, for the Largo and for the Allegro assai that closes this sonata, and which is one of the most lighthearted pieces in the Sonatas and Partitas.