Dennis asked:

 >>That is: 8 of the prelude/fugue pairs need a Db/C# at the same time,
 >>because the music modulates so far that it needs both.
 >
 >But do the pieces in question really need a well-tuned Db and C#
 >(though obviously not "at the same time")? If the C# is a long value 
in > a consonant tonic chord, and the Db a short passing dissonance, for
 > instance, then the music doesn't really need both? A good C# is not
 > only enough but preferable. Could you give a few examples of pieces
 > that need consonant enharmonic pairs?

Book 2, F major fugue.  Db major triad on the downbeat of bar 75, and 
big Bb minor chord on the downbeat of 87; A major triads in bar 21 of 
the prelude, and at several other places in both prelude and fugue.

Book 2, Bb major prelude.  Db-F 10th struck together on a strong beat of 
bar 63, and plenty of other Dbs elsewhere (and in the fugue); sustained 
C# in bar 42 where it's tonicizing D major on the next beat.

Book 2, G minor fugue.  A major in bar 31; Dbs in the decoration of Ab 
major, bar 62.  A stepwise alternation of Eb with a too-low Db (as C#) 
would sound odd there, where the Eb is the tonic of bars 61-63.  The 
prelude also uses both notes: C# at several places leading to D minor, 
and Db at one (admittedly short) spot going into F minor.


Brad Lehman