Because I chose not to approach this project in chronological (or logical) order, I have spent a lot of time flipping through the books of the WTC over the last 2+ years, fishing for the next piece to tackle. This little prelude is one that I tended to flip past. Sandwiched between the prayerful (and approachable) A-flat Major fugue and its masterful 4-voice companion fugue, it looks like an underdog on the page. Difficult to imagine Bach himself invested much in it. But with only 5 pieces left in this project, I had to do it and found, not an underdog, but work of perfect contrapuntal economy.
Bach applied mathematical structure to his music and had a particular affinity for 3s. (Holy Trinity thing.) This is crazy, but here goes: G-sharp is the 9th of the 12 semitones. Divisble by 3. Naturally, it follows that the number of this prelude—18—is also divisible by 3. 18/3=6. It’s in a triple meter, 6/8. The subject consists of 9 notes and is stated 10 times right-side up, 5 times inverted. 15 is divisible by 3, and the ratio of subject to inversion is 2/3. The 9-note subject is then sliced into three 3-note chunks and used throughout, in various permutations, as episodic material and to underlie the subject. In the hands of your average serialist composer, mathematics like this would have been dehydrating, but this is Bach.