(Bach to Basics, if you are inclined toward musical humor)
A prelude is what it sounds like: an introductory composition. There are no set forms for preludes, and we see a wide range of forms, styles, and characters in the preludes of the WTC.
A fugue is a more structured work. You can read the Wikipedia article about it if you want more details, but here’s essentially how it works: you start with a single voice that opens with the initial “subject” in the tonic, or home key. One to four more subsequent voices are quickly introduced with their own iteration of the subject. Once all the voices have entered, we cadence and are done with the exposition.
Next, there’s usually an “episode” in which the subject is not present at all. It’s typically in this area where we start getting the sense that we are harmonically moving away from the tonic. In terms of where you go, convention dictates a couple of options: follow the circle of fifths and end up in the dominant key (main “away” area, harmonies built on the fifth note of the tonic scale — e.g. if the tonic is C Major, the dominant would be G), or slide into the relative major/minor key area (keys that share the same key signature — e.g. C Major and A Minor have no sharps or flats, and only use white notes on the keyboard).
In this section, a lot can happen to the subject, including:
- Augmentation: the note values of the subject are doubled (e.g. a half note becomes a whole note)
- Diminution: the note values of the subject are halved (e.g. a half note becomes a quarter note)
- Inversion: the subject’s intervals are flipped (e.g. if the original subject started with an ascending fifth, an inverted subject would begin with a descending fifth)
- Stretto: stacked entrances of the subject (e.g. a second statement of the subject will begin before the first subject is completely finished)
- Fragmentation: only a small part of the subject is present
Occasionally several of these techniques will be used at once (e.g. a statement of the subject that is both in diminution and inverted). Eventually, we get back to the tonic, almost always with a restatement of the subject in the original key.
While fugues follow some loose conventions — there are certain things that will nearly always happen — they don’t have the formal structure of binary form (A:||B:||) or rounded binary/sonata form (A:||BA:||).
Finally, the pairing issue.
When I was a music student, I was always assigned preludes and fugues together, as a pair. Asking about why I always needed to learn the two together, answers varied but generally fell somewhere in the realm of “well, it’s convention so you probably should do them both.” I never found that to be a particularly satisfying explanation. Having played quite a few of these at this point, I think that there are generally three broad categories of pairing:
- Clearly musically paired, with musical ideas carried over from the prelude into the fugue.
- Complementary pairs, where they are musically related in terms of character or texture, but don’t have strong underlying unities beyond that.
- Basically unrelated.
From a logistical standpoint, it’s worked better to record each piece on its own. However, for the pairs that seem to go with each other, I have knitted the prelude and fugue together in the space I believe is referred to as “post-production.”