The Bb minor prelude is written in polyphonic three-part texture. Two motives pervade the piece - one substantial, appearing much like a "theme", the other short and versa- tile, including inversion and multiple sequential patterns. One can therefore describe this prelude as composed along the lines of a three-part invention.

There are several distinct cadential patterns which help in understanding the structure of this piece. The following six sections emerge:

Two substantial structural analogies determine the layout of this composition:

This prelude is conceived in rather lively basic character. Evidence for this interpretation can be found in the simple rhythmic pattern which consists predominantly of eighth-notes and quarter-notes, and in the broken-chord pattern which characterizes the main motive (see e.g. M: bars 1m-3m). The alla breve indication of the time signature further supports this decision. The tempo may thus be very fluent; the half-note beats should be perceived as swinging but unhurried.

Articulation contains a light legato touch in the eighth-notes and gentle non legato in the quarter-notes. Exceptions occur only in the keynote / leading-note / keynote formulas which must be legato; see L: bars 1-3, M: 8-10, U: 14-16 etc..

No ornament is indicated in the printed score. Nevertheless, it is a good idea to add one embellishment which would have been understood as implied by any performer of Bach's time. The dotted note on the middle beat of bar 82 represents, together with the anticipation of the resolving keynote (see upper voice: A-Bb-Bb) a typical closing formula which was conventionally decorated with a mordent. This mordent commences with the upper neighboring note Bb and contains two shakes, stopping short just before the seventh eighth-note of the bar.

One feature which should not be left unmentioned are the pedal notes in the final section. Bar 73 witnesses the beginning of an extended F in the bass which constitutes a four-bar long dominant pedal. A keynote / leading-note / keynote figure in the tenor (see bars 73/74) introduces the dominant six-four chord on the downbeat of bar 74 which then continues throughout bars 75/76, alternating with the dominant-seventh chord on the middle beats. In bar 77, the upper voice takes over the pedal note function with an extended Bb which is retained until bar 80d, before giving way to the melodic line which had been heard similarly at the end of the third harmonic section.

While this prelude's design is thus very clearly outlined, both by the two motives and the cadential closes mentioned earlier, there is another possible interpretation which should be mentioned. The prelude can be read as an - albeit somewhat irregular - three-part fugue, with M1 as the subject. Under this assumption, bars 1-42d would present the exposition, with subject entries in M, U, L and (redundant) U; bars 42-55d then provide the middle section, conveniently characterized by entries which are shared between two voices, and bars 55-83 embody the final section with two more entries in U and M. The striking correspondences between bars 1-16d and 55-70d on the one hand, bars 36-40m and 77-81m on the other support this concept of a ternary layout. The following sketch shows the prelude in Bb minor read as a fugue (ex. 43).

Commencing on the downbeat of a three-two bar, the subject of this fugue extends over four bars - including three rests - before closing on the downbeat of bar 5. The pitch pattern displays predominantly small intervals, interspersed only with two high-tension intervals (see the tritones Eb-A in bar 2 and Gb-C in bar 4). The rhythmic pattern contains three different note values - half-notes, quarter-notes and eighth-notes.

Of particular impact are the rests, all three of which are equal in duration (one quarter-note each) and fall on the first half of one of the weaker beats (see bars 1 and 2, beat 3; bar 4, beat 2). These interruptions in the melodic flow pose an important question regarding the structure of the subject: Do all or only some of them mark the end of a subphrase?

On the basis of this observation, the varied and slightly extended sequence should also be taken as a larger unit, and the rest in bar 4 therefore interpreted as another tension sustaining rest.

The harmonic background to the subject, although somewhat tedious to make out among the highly chromatic lines, supports this concept. As the final entry (see bars 96-100) shows particularly clearly, the two subphrases are harmonized in striking similarity. Of great importance, for the understanding of the rests and of the subject as a whole, is the fact that Bach seems conscientiously to avoid a perfect cadence at the end of the first subphrase but substitutes the expected tonic instead - if only by a major chord (ex. 45):

Combining these findings - on the one hand, the undoubted layout in two corresponding subphrases, and on the other hand, the equally valid observation of a relaxing gesture at the beginning of the fourth bar - into a dynamic interpretation leads to the following concept: Within the first subphrase, the tension increases during the four- note ascent, finds its climax on the downbeat of bar 2 in a vii⁷ chord, and diminishes during the return to the keynote. As the note before the tension-sustaining rest is simple and straight-forward in its direction towards the subsequent one, no interruption of the crescendo is desirable. Within the second subphrase, the tension rises similarly throughout the (ornamented) ascent until the Gb on the downbeat of bar 4 which is again harmonized by a vii⁷ chord. The rest, which occurs here not during the ascent but after what is perceived as the climax, is preceded by two ornamental notes. These present a small relaxation - as it were on a secondary level - before the Gb is picked up and a proper decline through falling tritone and (ornamented) resolution is launched.

In the competition between the two subphrase-climaxes, the second one clearly wins.

The entries of the subject in this fugue follow an extremely regular pattern. There are altogether twenty-four statements; only the two final ones are slightly shortened (see the asterisks).

(ex. 46)

The subject's real answer is either identical with the original (see T: bars 17-21) or differs only in its major-mode ending (see e.g. S: bars 5-9). Other modifications concern the end of the phrase which is twice shortened (see the two final entries) and three times extended into a weak-beat ("female") ending (see S: bar 9, B: bar 15, S: bar 56).

Exactly half of the statements are inverted, and the focus on a symmetrical design is even further emphasized with regard to strettos and parallels:

Despite the extraordinary emphasis on the subject itself, Bach invents one regular counter-subject as well as - in lieu of a second contrapuntal complement - a rudimentary figure.

The fugue encompasses thirteen subject-free passages. Due to the fact that the endings of subject and counter-subject do not coincide and that tight strettos, involving only slightly-shifted entries and beginnings of subject statements, are very frequent, there are several cases where primary and secondary material overlap.*

* The most consequential cases in which episode material overlaps with subject statements are:

As it turns out, the final portion of E³ (see bars 25-27d) is the only episode segment that is not distinguished by very particular motivic features.

This fugue represents basically calm character. Although the time signature indicates that each half-note equals an undivided beat, these beats should be stately and composed. The relative tempo of the prelude to the fugue is a complex one; it can be attained in either of two different mental processes:

(Approximate metronome settings: half-notes in the prelude = 88, half-notes in the fugue = 66.)

The basic articulation in this fugue is legato. Several important exceptions do, however, occur:

The design of this fugue is so ingeniously laid out that it requires no sophisticated analysis. There are five sections, each encompassing four subject statements (one in each voice) as well as one or more intermittent episodes and a closing one, and a coda with another set of four subject statements and the final episode.

There is a stunning analogy between this layout and that of the D^# minor fugue from the first volume of the Well-Tempered Clavier (compare chapter 8 of this book).

The development of tension in this fugue presents a long, extremely powerful increase launched in two consecutive thrusts. Section II surpasses section I just like section IV exceeds section III. Within the first pair of sections, all episodes are decreasing. With- in the second pair, by contrast, the intermittent episodes express no clear dynamic intention (E⁶, E⁷, E⁹) while the closing passages either embark on a relaxation which is later converted into a last-minute increase (E⁸ ), or produce a straight-forward crescendo.

It is this difference in the dynamic impact of the episodes which guarantees a progressive motion in sections III/IV despite the structural correspondence with sections I/II. Section V therefore begins on a dynamic level which is higher than that of the preceding sections. Its intermittent episode follows up on this development by presenting two bars of ascending sequences which also profess an increasing gesture. Only then does the tension abate somewhat towards the second mixed stretto; the fact that this marks the return to the home key enhances the effect of relief that is felt on this very high level of tension.

After a harmonic as well as dynamic release, it would be natural to expect a further completion of the curve in a resolving cadence. (Interpreters should uphold this "promise" as long as possible, and not give away the pending interrupted cadence too early - this would considerably impoverish the glorious conclusion of this fugue.) The double stretto of the coda adds a structural surprise to the harmonic one, by topping a process which seemed already to have passed its peak with a final climax.