This Prelude is built upon the following figures (Bar 1: Theme I, Bass; Theme II, Treble), which change places with each other, at the distance of one bar, thoughout the greater part of the movement.
Bars 1-16: Period I. Starting in A minor, it moves to the Dominant Minor, E (Bars 4-5), and touches A minor (Bar 6), the Relative Major, C (Bars 7-8), the Subtonic Major, G (Bar 9), and the Subdominant Minor, D (Bars 10-11), before returning to the Tonic (Bar 12) to end with a Half Cadence on the Dominant Major, E.
Bars 17-32: Period II. Passes through various keys before reaching the Dominant Minor at Bar 25. From here it works its way back to the original key to end with a Perfect Cadence on the Tonic Minor, A.
This Prelude is divided into two parts of equal length, separated by the double bar, and is built mainly upon two Themes, in length a little more than a bar each, possessing strong characteristics, and written in Double Counterpoint with each other. Foe example, the Treble of Bar 1 becomes the Bass of Bar 2 and vice versa. (The immediate working of the Themes in Double Counterpoint takes up two brs – i.e., one bar for the statements, and another to show them placed above or below each other.) Excepting in one instance at Bar 28 (Bass part), the two Themes always appear braced together; eight recurrences being found in each division of the movement. The Connecting or Episodal work placed between the statements of the two themes in the Attendant keys is taken mainly from Bar 3, used in the first part of the Prelude by direct, and in the second part by contrary motion. These Episodal bars are Nos. 3, 6, 7, 10, 12, and 14 in the first part, and 19, 20, 23, 24, 27, and 29 in the second part.
At Bars 4 and 5 the Themes appear in the Dominant Minor (E), covering the same ground (two bars) as at the commencement. When Bar 8 is reached, however, the ground occupied in the key for the time being is of half the original duration, one bar sufficing instead of two, because Double Counterpoint is not immediately brought to bear upon them. This frequently occurs after this point.
The great characteristic of Period II is its employment of both Themes by Inversion, and the ascendancy of this device in this part of the Prelude. When Bar 25 is reached, we meet with both Themes again stated in their original form, but in the very next bar they are used by Inversion. Only when two bars off the end are they again brought in in their original form, to give additional unification to the piece, and to help to clench its conclusion firmly in the Tonic key.
The device of Imitation reigns supreme in this Prelude, being found in one shape or another in every bar, except the final bars of the two great divisions of the movement. It also shows some of the most closely-knit and ingenious workmanship, perhaps, in the whole collection.
Bars 1-3: Subject in Bass [A minor].
Bars 3-5: Tonal Answer in Alto, with Counter-subject in Bass [E minor].
Bars 5-6: Codetta, modulating from A minor to C major.
Bars 6-8: Subject in Treble, with Counter-subject in Alto [A minor].
Bars 8-9: Episode I, modulating from A minor to C major.
Bars 9-11: Subject in Bass, with Counter-subject in Treble [C major].
Bars 11-13: Episode II, modulating from C major to E minor.
Bars 13-15: Answer in Treble, with Counter-subject in Alto [E minor].
Bars 15-17: Episode III, modulating from E minor to A minor.
Bars 17-19: Subject in Alto, with Counter-subject in Bass [A minor].
Bars 19-21: Episode IV, modulating from A minor to D minor.
Bars 21-23: Subject in Treble, with Counter-subject in Bass [D minor].
Bars 23-25: Episode V, modulating from D minor to A minor.
Bars 25-27: Subject in Bass. Counter-subject absent [A minor].
Bars 27-28: Coda [A minor].
Exposition: Bars 1-8.
Coda: Bars 27-28.
- This Fugue has a “tonal” Answer, and would be called a “tonal” Fugue. The alteration in the Answer, for the sake of tonality is made from the first note to the second.
- A Counter-subject is absent on one occasion, Bars 26-27.
- The Subject at Bar 17 has its first note altered. At Bars 9 and 25 a note is inserted – a Passing Note – between the first and third notes. These slight alterations are made in order to make the workings even more elegant.
- The Episodes are very regular as to position, one Subject (or answer, as the case may be) standing between each. Their modulations are also very simple. The material for them all is drawn from three sources, (1) the last four notes of the Subject, (2) the early part of the Counter-subject, and (3) the detached demisemiquaver (thirty-second note) figure of the Codetta.
- Episode I (Bar 8). This is constructed in its first bar from the four last notes of the Subject, which here are assigned to the Treble, the Contralto being formed from the detached figure of the Codetta, the Bass part being complementary.
- Episode II (Bar 11) is built upon the last notes of the Subject, the last notes also of the Counter-subject being added to it.
- Episode III (Bar 15) in its first bar has the detached figure of the Codetta in the Treble and Bass, and reminiscences of the last notes of the Subject in the Alto. Its second bar employs reminiscences of the early part of the Counter-subject.
- Episode IV (Bar 19) is formed in the Bass upon the last figure of the Counter-subject. Upon this Bass a new theme is proposed by the Treble (Bar 19) which is imitated by the Alto in the fifth below.
- Episode V (Bar 23) is mainly built upon the last four notes of the Subject, and the detached figure of the Counter-subject.
- Bars 25-26 are of a Bravura character.
- The Coda is built upon suggestions of the latter part of the Subject and Counter-subject.