Bars 1-16: Period I. Starting at once with Theme I in D sharp minor, it modulates to the Relative Major, F sharp (Bars 8-9) before ending with a Perfect Cadence in the key of the Dominant Minor, A sharp.
Bars 17-36: Period II. Starting by bracing Themes I and II at the double bar. From starting on the Dominant, touches F sharp major (Bar 19), the Subdominant Minor, G sharp (Bars 23-24), before returning to the Tonic minor through a descending Sequence (Bars 25-28).
This Prelude is divided by the double bar into two halves of nearly equal length, and the device of Imitation is largely employed throughout.
It is started by the proposition at once of Theme I, which is mostly employed in the first half of the Prelude, being frequently imitated at the Octave, at the distance of one bar. Theme II first appears at Bar 2 in the Treble, and is imitated by the Bass at the fifth below at Bars 3-5. This Theme also appears in the second half of the movement at Bars 21-23, being here proposed by the Bass part and imitated by the Treble at the distance of a fourth.
Theme III makes its first appearance at the commencement of the second half of the Prelude (Bar 17), where it at once appears in company with Theme I, as it does on several occasions afterwards (see Bars 19, 32, 33, 34), where it is sometimes above and sometimes below Theme I.
The first and third Themes are written in sharp contrast with each other, Theme III being specially characteristic.
Period I is capable of a momentary division at Bar 9, where there is a Perfect Cadence in the key of the Relative Major, F sharp.
Period II mometarily divides at Bar 24 with a Cadence in G sharp minor, and at Bar 28, where there is a Perfect Cadence in the key of the Tonic minor (D sharp)
Bars 35,36 are a transposition of Bars 15-16 into the key of the Tonic, this imparting an unifying influence to the Prelude.
Bars 1-3: Subject in Alto [D sharp minor].
Bars 3-5: Real Answer in Tenor, with Counter-subject in Alto [A sharp minor].
Bars 5-7: Codetta, modulating from A sharp minor to D sharp minor.
Bars 7-9: Subject in Bass. Counter-subject in Tenor [D sharp minor].
Bars 9-11: Answer in Treble. Counter-subject in Bass [A sharp minor].
Bars 11-15: Episode I, modulating from A sharp minor, through D sharp major, to G sharp minor.
Bars 15-17: Answer (altered) in Bass. Counter-subject in Treble [G sharp minor]
Bars 17-19: Subject (altered) in Alto. Counter-subject absent [F sharp major].
Bars 19-21: Subject (altered) in Tenor. Fragment of Counter-subject in Bass [G sharp minor].
Bars 21-23: Subject in Treble. Counter-subject in Tenor [D sharp minor].
Bar 24: Stretto I, first limb of Subject in Tenor [G sharp minor].
Bars 25-27: Subject in Bass [D sharp minor].
Bars 27-29: Subject in Treble [C sharp minor].
Bars 30-32: Subject in Alto [F sharp major].
Bars 32-34: Subject (altered) in Tenor [G sharp minor].
Bars 34-40: Episode II, modulating from D sharp minor, through A sharp minor, and back to D sharp minor.
Bars 40-42: Subject in Bass [D sharp minor].
Bars 42-43: Codetta, confirming the key of D sharp minor.
Bars 43-45: Subject in Treble, with Answer in Tenor simultaneously by Inversion [D sharp minor].
Bars 45-46: Coda.
Exposition: Bars 1-11.
Coda: Bars 45-46.
- This Fugue has a “real” Answer, and would be called a “real” Fugue.
- The Counter-subject is not used regularly throughout, its last appearance being in Bar 21, after which it entirely disappears.
- After the Exposition, the Subject is used four times in a materially altered form, and five times in its original shape. The altered form, which first appears in Bar 15, consists in exchanging the skip of a minor third for the original minor second, from the third note to the the fourth, and thus continuing the melody at this lower pitch until the end. By this means the above-mentioned Subject is made to end in G sharp minor instead of A sharp minor. At Bars 17, 19, and 32 much the same kind of alteration takes place.
- At Bar 40 and onwards the SUbject in the Bass is homophonically accompanied, which is unusual (Compare Fugue XVIII, Book 1, Bars 8-11).
- At Bar 43 the Subject in the Treble and its Answer in the Tenor by Inversion are employed simultaneously.
- The Episodes are built upon the Subject. A new figure, however, appears in Episode II at Bars 38-39 together with a passage from the first Codetta.