Bach WTC Book 1 No. 8

My son said that what he has heard is that Bach wanted to have pieces in every key without having to re-tune; the intention here is to highlight that feature, i.e. having two pieces in keys that would be different in equal temperament but the same in well temperament. 

beyond the key: if by chance you play or practise on a digital instrument then personally i highly recommend to (at least from time to time try out to) play Bach pieces at a = 415Hz. They sound more beautiful to me with that pitch. 

if not your visceral auditory response, could it be the key fingering, some which can “feel” more suited?

It's fascinating to me (and impressive) that some of you have such finely tuned pitch-awareness that you hear a difference when played in A415 vs. A440.  I'm not sure I would ever notice, unless played side by side. But my understanding is that if you played these pieces on an untempered instrument such as a violin, an A# and a Bb for example would be different notes.

Although these are solo piano pieces, Bach may have had "violin keys" in mind in some places, or perhaps he felt that the "sharp" version  better reflected the tones in his mind than the "flat".  I've also read somewhere that Bach's 'Well-Tempered' does not necessarily mean our version of 'Even Temperment'.  So the true tones available on the keyboard might have seemed more appropriate in a sharp key or flat key to him, depending on the surrounding notes and context..

I first learned the Eb Minor Fugue Book 1 from the Schirmer edition (Czerny editor), which, interestingly enough had actually been written out in Eb Minor rather than D# minor.  (transposed? by Czerny himself? one of Bach's student/copyists? a Schirmer in house editor?) I re-learned it later from an Urtext in D# minor, which wasn't very easy, but certainly a good mental exercise...

And FWIW, as I understand it, the C Sharp Major Prelude in Book 2 was based on a version originally written out in C Major. So there's precedent for transposing from a more common key in order to fill out the catalogue of 24 major-minor keys.  Something that is already written out in E minor would be much easier to transpose into Eb Minor than D# Minor -- and would reduce the possibility of error by those whom he may have charged with actually writing out the notation. Just change the key signature and you are almost done, except for some accidentals.  Convenience might certainly been a consideration to someone who was writing 2 or 3 masterpieces per week, while also rehearsing the choir, carrying a full teaching load, and facing down organ-playing challenges from whatever hotshot competitor was in town, not to mention tending to 20 children!

I believe there is some confusion here. Bach never composed the Fugue No. 8 of the Well-Tempered Clavier in D-sharp minor but rather in E-flat minor. The Well-Tempered Clavier was Bach's concept to prove that one could play in all 12 keys of the keyboard, both major and minor. Versions that transpose this fugue, as well as others like the C-sharp major fugue to D-flat major, were only created by editors to simplify reading. This demonstrates the lack of solid theoretical foundation seen in recent decades, as during the era of great composers, including Bach, transposing of a piece were done immediately, playing in different keys to showcase their profound understanding of music. I cite the example of Beethoven, who, when presented with the young Liszt, asked the child to play the C minor fugue from the Well-Tempered Clavier in A-flat minor, which the prodigy Liszt accomplished with ease, impressing the composer from Bonn.

This reinforces the point that great composers possessed a deep mastery of music, allowing them to easily transpose and adapt pieces, something often overlooked in modern interpretations.