This Prelude is built on the characteristic figure as seen in the first three bars above.
Bars 1-16: Period I. Starting in the Tonic, F major, it touches the Dominant, C, (Bar 3), then G minor (Bar 6) before returning to the Tonic (Bar 8). Ending with Perfect Cadence in the key of the Dominant, C.
Bars 17-32: Period II. Starting in C, then touching D minor (Bar 22), C major and F major (Bars 24-25). Ending with Half Cadence in the key of the Relative Minor, D.
Bars 33-46: Period III. From D minor it transitions to the Dominant, C, stopping in F major on the way (Bar 41-42). Ending with Perfect Cadence in the key of the Dominant, C.
Bars 46-49: Short connecting passage from C major to A minor.
Bars 49-56: Period IV. Touches D minor (Bar 50) before ending with a Perfect Cadence in the key of the Mediant Minor, A.
Bars 57-72: Period V. Starting in F major, it touches the Subdominant Major, B flat (Bar 64). Ending with a Perfect Cadence in the key of the Tonic, F.
Two Themes are employed in this Prelude. Theme I which is of considerable extent and is necessarily divided among different voices as it runs along its course to completion. This Theme is interlaced incessantly in the most ingenious manner throughout the whole of the movement.
Theme II is a gently ascending figure in crotchets (quarter notes), and in the sharpest contrast to Theme I does not make its appearance till Bar 49 (Tenor part) when it is combined with Theme I.
Theme I appears at the commencement of every Period.
At Period I it appears in the Tonic key (F).
At Period II it appears in the Dominant (C), and transplants for ten bars to opening material into the Dominant key.
At Period III it appears in the Relative Minor (D), and transplants into it the early part of the opening material, after which it works on with a free version of the same.
At Period IV it appears in the Mediant Minor (A), again with a free version.
At Period V it appears as a Recapitulation and for six bars reproduces note for note the opening matter. The remaining bars of the Prelude are devoted mainly to establishing firmly the Tonic key upon the material of Theme I shared amongst the different voices.
The Periods are not capable of subdivision.
The “form” of this movement is probably one of those experiments in structure which Bach worked out and perfected for himself. The harmonic centres – as shewn at the various Periods – are very clearly indicated and systematically distributed; great uniformity is observable in the thematic material, and the subject-matter is introduced in clear identity with the opening at those particular points where the indication of the commencement of a new key block is required. The power of every chord as a unit in the great tonal centre for the time being is carefully considered, and its force of curve to the side or to that fully adjusted. The ground used for action is first effectively staked out by the great harmonic bases, and every chord within this area is made to pull its proportioned weight up to its ultimate goal. The coherence, consistency, and unification of the whole is never lost sight of, and – though quite out of the bounds or suggestions of Sonata form – a more perfect picture it is impossible to imagine.
Bars 1-5: Subject in Treble [F major].
Bars 5-9: Tonal Answer in Alto. No Counter-subject [C major].
Bars 9-14: Codetta, modulating from C, through G minor, B flat major, D minor and back again to F.
Bars 14-18: Subject in Bass [F major].
Bars 18-21: Codetta, modulating from F to C.
Bars 21-25: Answer in Bass [C major].
Bars 25-52: Episode I, modulating from C, through various keys transiently and back to F.
Bars 52-56: Subject in Alto [F major].
Bars 56-66: Episode II, modulating from F to B flat.
Bars 66-70: Subject in Bass [B flat].
Bars 70-85: Episode III, modulating from B flat to F.
Bars 85-89: Subject in Treble [F major].
Bars 89-95: Subject in Bass [F major].
Bars 95-99: Coda [F major].
Exposition: Bars 1-18.
Tonic Pedal: Bars 61-65.
Dominant Pedal: Bars 76-82.
- 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 third to the fourth note and from the fourth to the fifth notes.
- The first Codetta is remarkable for its length. It is written in a free ascending sequence, it also furnishes, almost exclusively, the material for the construction of the Episodes.
- The same part (Bass) which entered with the Subject (bar 14) enters also almost immediately with an Answer (Bar 21). This is somewhat rare, the general rule being that a different part to the last employed should enter next.
- In the last two entries of the Fugue (1), that for the Treble (Bar 85), the Subject is chromatically altered for the moment at Bar 87, and (2) in that for the Bass (Bar 89) the first limb of the Subject is four times repeated, one note higher each time.
- This Fugue is remarkable also for the small area alloted to the Thematic work as compared with that assigned to the Codettas and Episodes; these latter occupying 62 while the former occupy only 34 bars of the entire piece. This the Episodal matter almost doubles that of the Thematic.
- The first Pedal (Bars 61-65) acts as a Dominant to the key of B flat, which is the ruling key at that point.
- Four Episodes:
- Episode I (Bar 25), up to Bar 29, aims at more fully confirming the key of the Dominant touched upon previously. Imitations in the two upper parts upon the last portion of the Subject are seen in Bars 30-34, the Bass having notes of greater value. From Bars 34-45 the material, style, and treatment are borrowed from the first Codetta. Bars 45-52 take material from both Subjects and the Codetta, the Bass part of Bar 45 showing the former and of Bar 46 the latter material.
- Episode II (Bar 56). This takes a short figure from the first Codetta and freely twists it round, using it by Inversion. Its latter portions stands on a Pedal.
- Episode III (Bar 70) is formed much the same as Episode II. For a moment there is free canonical imitation between the Treble and Alto (Bars 72-76), but it breaks off in preference to a freer treatment. Part of this Episode also stands on a Pedal.