Bach: Prelude and Fugue No.21 in B♭ major, BWV 890 Analysis



Bach Prelude and Fugue No.21 in Bb major BWV 890 Analysis 1


This Prelude is built mainly upon the figure seen in the first three bars above, which is frequently used by contrary motion as the movement proceeds.

The two following themes are also very notable:

  1. Theme II (Bars 8-9).
  2. Theme III (Bars 13-15).

and the blocks of work which they appear and are in fact the main-stays in the first part of the movement, are bodily shifted into different keys in the second part (See “Remarks” below.)

Bars 1-32: Period I. Ending with a Perfect Cadence in the key of the Dominant, F; double bar.

Bars 33-76: Period II. Ending with a Deceptive Cadence on the Third Inversion of the Dominant 7th.

Bars 76-87: Coda. Forming a very effective and brilliant conclusion to the movement.


This Prelude has two main divisions separated by the double bar. The number of parts employed in the course of its construction varied, mostly three parts are used, but on several occasions only two.

Three important blocks of work which are employed in the first part of the movement, founded on the Themes given above, will be found also in the second part. They are (1) the Opening bars as a Recapitulation in the Tonic at Bar 49 and onwards. (2) Bars 9-13, which in the first part of the movement are working in the key of the Dominant, reappears at Bar 53 and onwards transposed into the Tonic. (3) Bars 13-17 are also transposed at Bars 37-41.

Both Periods are capable of subdivision.

Period I divides into five sections:

  1. At Bar 4, where there is a Half Cadence on the Dominant of the original key.
  2. At Bar 8, where there is a Half Cadence on the Dominant of F.
  3. At Bar 13, where there is a Half Cadence on the Tonic.
  4. At Bar 17, where there is a Perfect Cadence in the Subdominant (E flat).

Bars 17-32 are not capable of subdivision.

The second half of the movement divides into four sections, which are longer and the work contained in them more massive than in those composing Period I.

  1. At Bar 48, where there is a Perfect Cadence in the key of the Relative Minor (G), leading to the Recapitulation of the Opening bars in the key of the Tonic.
  2. At Bar 64, where there is a Half Cadence on the Dominant of B flat.
  3. At Bar 76, where there is a Deceptive Cadence on the third Inversion of the Dominant chord, followed by a momentary pause previous to the Coda

The principle material forming the Opening of Period II is taken from Theme I freely used by Inversion.

The Coda, which is mostly in two parts, is built chiefly on fragments of Theme I, tossed about in similar and contrary motion simultaneously.

This Prelude is the most advanced in “form” of any of the “Forty-eight,” pointing as it does in the most unmistakable manner to the Modern Sonata form. The Second Subject in the key of the – the complementary key – begins at Bar 9, its definiteness of entry being notable. This is transposed into the Tonic key inf the second part of the movement. The part immediately following the double bar starts without any very direct allusion to former material; a free rein is almost at once given to modulation, and the treatment of the whole Section points to the modern “Development” or “Free Fantasia.” Next, the Opening subject slightly curtailed, appears as a Recapitulation, and then the material is drawn forcibly onwards, until it meets a Deceptive Cadence on the heaviest Inversion of the Dominant chord, and the brilliant and effective Coda which follows is prophetic of the great Codas of Beethoven.



Bach Prelude and Fugue No.21 in Bb major BWV 890 Analysis 2



Bars 1-5: Subject in Alto [B flat major].
Bars 5-9: Tonal Answer in Treble. No Counter-subject [F major].
Bars 9-13: Codetta, modulating from F to B flat.
Bars 13-17: Subject in Bass [B flat].
Bars 17-21: Episode I, modulating from B to F major.
Bars 21-25: Answer in Bass [F major].
Bars 25-32: Episode II, modulating from B to F.
Bars 32-36: Answer in Alto [F major].
Bars 36-40: Episode III, modulating F to B.
Bars 40-44: Subject in Treble [B flat].


Bars 44-47: Episode IV, modulating from F major to G minor.
Bars 47-51: Subject in Bass [G minor].
Bars 51-54: Episode V, modulating from G minor to E flat major.
Bars 54-58: Subject in Alto [E flat major].
Bars 58-63: Episode VI, modulating from E flat major to C minor.
Bars 63-67: Subject in Treble [C minor].
Bars 67-78: Episode VII, modulating from C minor, through G minor to B flat major.


Bars 78-82: Answer in Treble (slightly altered) [B flat major].
Bars 82-93: Coda [B flat major].


Exposition: Bars 1-17.
Counter-exposition (incomplete): Bars 32-44.
Codetta: One.
Stretti: None.
Episodes: Seven.
Coda: Bars 82-93.


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 in the fifth and sixth notes.

The Answer ins given in a “real” form on two occasions in Bars 21 and 32.

There is no Counter-subject in the strict sense of the term, but, as if to compensate for this, two short themes are made use of, and they appear for the first time in Bar 33 in the Treble and Bass. With the subsequent entries of the Subject and Answer these themes are used in Double Counterpoint of both the 10th and 12th.

The Episodes are simple in construction, being all formed from the Subject. Their position also is very regular, one Subject (or Answer, as the case may be) standing between each.

The Coda is constructed upon the first three bars of the Subject, and also from the two themes above-mentioned, its last five bars being taken from Bars 28-32, transposed from the Dominant into the original key.