One set of pieces that has always intrigued me in terms of tuning is 
Pachelbel's Hexachordum Apollinis (1699), for organ or harpsichord 
(see full title, reproduced at IMSLP, Google, Wikipedia etc).

Suffice it for my purposes to note that the six sets of variations 
are, following the hexachord, in the keys of D minor, E minor, F 
major, G minor, A minor... until the last, which is in F minor 
(though with a B-flat signature). This last is named "Aria 
Sebaldina", and the name and the key must have some special 
significance, as Pachelbel was organist of the church of St. Sebaldus 
in Nuremberg. (Or did he just transpose it up a tone, pretending by 
means of the key signature to satisfy the hexachord title?)

There are, in my opinion, no cases of deliberate "poison notes" in 
these works; i.e. notes that are intended to sound bizarre by not 
fitting the tuning. Indeed, the pieces are firmly diatonic throughout 
and, again in my opinion, seem to be conceived for the pure thirds of 
1/4 comma meantone tuning. The variations in D minor, F major, G 
minor and A minor fit right in; but the three D-sharps of the E minor 
variations do not sound good when tuned as the normal E-flats of MT, 
and I retune them on the harpsichord to be consonant with the B naturals.

The last set is even more extreme, requiring A-flats and D-flats from 
the very beginning, which sound quite out of place if tuned as 
G-sharps and C-sharps. (Even allowing that Pachelbel avoids the third 
and fifth degree in bar 8 of the theme, though not elsewhere, there 
is nothing more disconcerting than a perfect cadence in A-flat that 
lands on three octave G-sharps!) Retuning the notes to be consonant 
with C and F respectively transforms the piece into a quite ethereal 
F minor, a world away from the usual experience of F-minor as an 
extreme funereal key. There is a further demand in the sixth bar of 
the the last variation, which indicates a single G-flat in alt.

The elephant in the room, of course, is the question of how Pachelbel 
himself intended the organ and/or harpsichord to be tuned.

By the way, the original engraving is beautifully done and playing 
from the facsimile is an excellent way of mastering soprano clef, and 
detecting the occasional obvious errors.

David

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