The piece is included in the Andreas Bach Book. Johann Andreas was Bach's nephew. He inherited the music book of his brother Johann Bernhard, who had studied with Bach for a long time, and signed it with the year 1754. It is currently housed in the Leipzig City Library. The opening 10-measure “Fantasia” appears only in this source. Regarding these arpeggios, for example, Chromatic Fantasia, BWV 903, measure 28, Bach's actual performance practice can be referenced. Furthermore, F. Liszt presented his own realization in his edited version (Liszt's arrangement is still available today). Bach sometimes notated only simplified chords, prompting the performer to complete them, but usually, as in the Chromatic Fantasia, he provided a solution. It is extremely rare for there to be no instructions whatsoever like this. Moreover, in sources other than the Andreas Bach Book, this section is omitted, and only the fugue is copied. Bach probably did not consider this state to be the final complete version. The 10 measures of chords might have been a sketch for a full-fledged prelude, or perhaps merely hints for improvisation.

The Fugue

The fugue is a lengthy work with a high virtuoso effect. The whole has the following logical structure: Subject exposition (A minor) - Development (E minor) - Development (D minor) - Development (C major) - Recapitulation: Subject exposition (E minor - A minor) - Development - Coda. The first development in the first half (E minor) gradually rises in range, the next development (D minor) broadly maintains the middle range, and the third development (C major) gradually lowers again.

The recapitulation and coda are each separated by descending arpeggios spanning two and a half octaves. The recapitulation begins with the dominant subject in the bass, and as the overall range lowers, the long-awaited tonic subject begins majestically. Subsequent developments connect fragments of the subject in short cycles, with harmonies rapidly changing, preparing for the subject exposition at the beginning of the coda. This marks the highest range for the subject throughout the entire piece. In the coda, the music alternates between high and low registers, re-exposing the subject before concluding.

Such shifts in range can achieve a three-dimensional effect if one utilizes the multiple keyboards of a harpsichord. On a modern piano, attention must be paid to changes in timbre and dynamics for each register, but the interplay using suspensions in the development sections is an idiom particularly well-suited to the piano. Sixteenth notes continue without pause; playing fast would allow for a display of brilliant technique, while proceeding slowly through the unusual 3/4 time for a fugue would allow one to enjoy the harmonic changes.