(con't from Part XXX)
As J.S. Bach has amply demonstrated, when we're composing there are many, many ways, once we've written a keyboard fugue, to "pair it" with a related prelude (photo).
A prelude and fugue, as a pair, do not have to spring from the same ideas; the vast majority of the preludes and fugues of Bach in fact show no thematic or rhythmic connections whatsoever.
But when they do (as in Bach's "Great" d minor Toccata and Fugue BWV 565 for organ) the listener senses an inner unity in the work [See blog, Bach d minor, Parts I-V].
When the fugue which follows the prelude has 4 voices, 2 countersubjects, and a free voice in the soprano somewhere that makes a pleasant melody line, we can use that same line as the theme for the prelude, building it into a piece with one theme; this prelude is then given the same 6-part treatment that Vierne taught his students for improvisation on a single free theme [See blog, Learning By Example].
If this procedure is followed the prelude theme, when it suddenly reenters later in the fugue during one of the expositions, is a novelty which lends a certain interest and sense of unity to the entire work; it seems in fact so cunningly woven into the fabric of the fugue that it almost disappears in plain sight.
In the real world of composing however, this is merely an illusion; it's not that cunning, it's just a matter of compositional technique with the fugue being written first and the prelude completely built from one of the fugue's free voices.
What's intriguing here is, even careful listeners may not at first recognize that the prelude's theme is hidden in the fugue; it appears in the fugue only once, and when it does, even though it's in the soprano line, it's virtually camouflaged by the busy melodic activity going on underneath it.
The score for Prelude and Fugue in a minor Op. 25 is an example of a prelude and fugue where the pair are completely unrelated except for one thing: the theme for the prelude is used one time in the fugue, in just one place, as a free voice which completes a 4 part texture [See menu bar, Free Stuff, Op. 24-25 subpage].
The Fugue is in 4 voices with 2 countersubjects and a subject 5 bars long which first enters in the tenor on the 1st scale degree and is answered in the alto on the 5th scale degree, as expected; a short passage of 2 bars allows a smooth return from the dominant to the home key in preparation for the 3rd entry.
The 3rd entry is in the soprano with the 1st countersubject in the alto and the 2nd countersubject in the tenor, again as expected.
The 4th entry is in the bass with the 1st countersubject in the soprano, the 2nd countersubject in the alto, and the free voice in the tenor, again, all as expected.
Additional entries of the subject in the expositions which follow proceed through all nearly related keys in this order: C Major, e minor, G Major, d minor, and F Major, with a return to the home key for the final entry); these expositions are each separated by 4 bar modulating episodes employing sequences based upon fragments of the subject or countersubjects, sometimes in inverse movement.
When the subject enters in d minor in the alto, the 1st countersubject enters in the tenor, the 2nd countersubject in the bass, and a free voice fills in the soprano to complete a 4 voice texture; it's this free voice which will become the theme for the Prelude.
The subject enters in the lowest octave of the pedals for the final entry, following many examples left to us by J.S. Bach; a short coda of 6 bars over a dominant pedal point rounds out the work which finishes with a trill on the penultimate chord; a 5th voice is added here and the piece ends in 6 voices on a Picardy third (major 3rd interval in the final chord of a piece written in a minor key).
As for the Prelude, it's constructed, once again, in the 6-part form which Louis Vierne taught his students to employ for improvisation on a single free theme [See blog, Learning By Example, Getting Started With Writing, Part XXXII; here the top line from the d minor exposition in the Fugue is used for the theme of the Prelude.
This Prelude begins with 2 bars introducing the first bar of the theme in inverse movement in the tenor line over a reiterated low A in the pedal; the theme then enters in the soprano line and is harmonized in 4 parts over the course of the next 7 bars, ending in a perfect cadence in the home key; this completes the 1st exposition of the theme.
Part 2 is a bridge section of 6 bars consisting of 2 short phrases of 3 bars each which outline the first bar of the theme, first right side up and then in inverse movement, and modulates to the raised submediant key (f# minor).
Part 3 is the 2nd exposition of the theme in f# minor; here again the theme enters in the soprano, but this time with the tenor line following it in imitation at the distance of a half bar at the interval of a 5th; this continues for 7 bars, is harmonized in 4 parts, and ends with a perfect cadence.
What follows is a section 20 bars in length which develops the first bar of the theme in inverse movement; this development section begins in the parallel major key (A Major) and modulates through the keys of C Major, Eb Major, d minor, and b minor before coming to stop which points back to f# minor.
Part 5 is a passage of 4 bars which functions as the preparation for the reentry; it consists of 2 phrases each 2 bars long which modulates from f# minor to g minor and finally points to the home key.
The final 6th section is the reentry of the theme in the top line, in the home key, which is harmonized again in 4 voices.
A coda of 8 bars concludes the piece; in this coda the theme is reiterated slowly in the pedal and segmented until nothing is left of it but a single tonic note; the work ends very quietly in the primitive gloom of the a minor home key.
The way this Prelude is put together, particularly the development section with its several excursions into distant keys and the employment of the raised submediant key in the 2nd entry, will repay careful study and may be used as a model for any number of similar works.
Prelude and Fugue in b minor Op. 26, for example, is closely related to this work both formally and thematically [See blog, Getting Started With Writing, Part XXXII].
(con't in Part XXXII)