Very interesting observation, Thomas.
Your argument makes sense in some way similar to the fuge in d#-minor:
Just repeating
The Fugue in d# minor (BWV 853) in WTC I is a good example how to determine a strong hypothesis for a transposition:
In measures 15-16 there is a simple scale going up to b#2. The b#2 is being held across the barline then jumps a septima below to c# and cadences (shortly) in F# major:
Eighth-notes, all in the second octave:
| b c# d# e# f# g# a# b#– | –b# c# [f#_]
The septima jump, otherwise a decisively doubtful solution (contrapuntally, melodically), was simply forced, because otherwise the standard WTC-1 compass in the treble (b2 = c3) would have been exceeded. This example shows Bach as the highly pragmatic composer he was.
If the original can be assumed to have been in d the somewhat awkward septima jump disappears. The result is:
| bb c d e f g a b | c3 c [f_]
Interestingly this piece has a subject and its contrapuntal treatment displaying a distinct touch of “old style”. It never exceeds the meantone range (keys from eb, bb… c#, g#), either, and can be played without problems on a fretted clavichord (d and a unfretted). It needs however a full chromatic compass in the bass.
My caveat for the fugue in b minor (BWV 867), though, would be the range of pitches (in the circle of fifths)
in g-minor: db, ab … f#, c#.
In a-minor: eb, bb … g#, d#.
The d sharp (in b-minor: e) never occurs in a consonant chord. The exception seems at first to be the 2nd half of bar 73, but there the d# is struck in the dissonance of the first half and the sound will have decayed until the entry of the second half of the bar. That entry is a sixth chord which is less sensitive to temperamental issues than root position. So, if te note would be tuned rather like an e-flat instead of an d sharp, it would not disturb the general harmonic impression. Fretting shouldn’t be a problem then either ...
Kind regards
Ibo
> Am 22.01.2017 um 23:47 schrieb Thomas Dent <[log in to unmask]>:
>
> Hi again,
>
> btw, I once wrote out the Bb minor P&F from 'Book 1' to play on a
> double-fretted clavichord (chromatic from C in the bass) in G minor. It
> seems to work quite well - noting that the tessitura of both pieces is
> pretty high - except for the one B natural in the bass (near the end of the
> Fugue).
> BUT then you notice that that note is part of a line which descends an
> octave in the middle of a bar, then a couple of bars later jumps back up
> the octave - so no contrapuntal content is lost by *not* descending the
> octave, the low B natural is not at all necessary - if anything the
> octave-down jump looks like an afterthought.
>
> Almost as if Bach wrote the piece in a lower key, then transposed it up
> into Bb minor, then noticed that this line could be given some extra
> gravitas for free in the higher key ..
>
> .. does this make any sense? I suppose A minor is the other obvious
> 'original' key, but somehow G minor feels more logically related, and there
> would not be any Ab/G clashes in an 'original' G minor version - i.e. it
> would be completely 'fret-friendly'.
>
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