Mr. Bach must have been thinking about a fugue from the previous WTCI (no. 14 in f# minor) when he wrote today’s fugue in Bb Major (no. 21) for the WTCII. The subject of today’s fugue includes a descending repeated note pattern for a couple of measures (circled in the uppermost score in the image), which is featured very similarly in the earlier fugue (circled in the lower score) although not as the subject, but the countersubject. Did he think about this countersubject for twenty years between the two books, and finally decide it was time to make that lovely line a proper subject?
It’s mysterious to me that every time I play today’s P&F in Bb, it’s as though I’ve never heard nor played it before — it just doesn’t stay with me, whereas once I hear or play the F# Minor P&F from WTCI, I can’t get it out of my head for days. I feel a little sheepish admitting such a strong preference for certain P&F’s, but there it is. Some summer of the future I will play today’s P&F and say to myself, “Ah! This one! I remember it!.” I hope.