Is it okay to mix accidentals when writing enharmonic notes in different parts?

Question

I had someone ask me the above question about "Dialogue" by Eugene Bozza:

The piece ends in octaves, written Ab in the 1st part and G# in the second part. It's definitely a little strange at first glance, but I pointed out to them how (especially in the last two lines) the first part uses almost exclusively flat accidentals, while the second part uses sharps. I talked about how, for the ease of reading, it makes more sense to have that last note match the 'mindset' that the player is in (sharps mode vs. flats mode) than to have the last note be written in the same way.

I think I generally got my point across, but I never formally studied part writing, so I figured I'd also ask here -- is my answer generally right? Are there general principles for when ease of reading overrules things like writing enharmonic notes in different parts?

Answer 1

In "Dialogue"

You're correct that it's for ease of reading, but your reasoning can go a step further.

Both parts are "in sharps" for most of the piece, excepting a few flats in Trumpet II. However, Trumpet I moves into flats for the final five bars for ease of reading. To remain in sharps, the fifth-to-last bar, for example, to maintain intervalic clarity, would require E#s and Fxs rather than Fs and Gs, because of the presence of D# (i.e., Eb).

Likewise, where Trumpet II rewritten in flats, a similar problem would occur.

In general

As far as general rules, here are a few scenarios in which enharmonic equivalents might be used simultaneously:

• If two parts are in different keys, then each would be written with its expected key signature, possibly resulting in enharmonically equivalent but differently notated pitches.
• Another scenario might be with parts respectively ascending and descending chromatically. Should they encounter the same enharmonic pitch, the ascending part would likely be written with a sharp, the descending part with a flat. This would similarly affect, say, 12-tone music scored with accidentals on every pitch, as needed.
• Transposing instruments can result in enharmonic equivalents for ease of reading. For instance, a piece in B major, written with five sharps for C instruments like flute, might be written in Db major (rather than C# major) for trumpets in Bb, because the former has 5 flats while the latter has 7 sharps.