What is the idea of "transposing by perfect unison"?

Question

As far as I understand, a "perfect unison" refer to two notes sounding the same pitch. Why both MuseScore and Sibelius 7 (I work with these two programs) offer a feature to transpose the up or down by "perfect unison"?

I tried to transpose a single A placed on the usual staff. When I select "augmented unison", the computer places the # in front of it (if to transpose up) or b (if to transpose down). Hence the augmented unison seems exactly one half tone distance.

However nothing happens when I try to transpose by "perfect unison", that makes sense taking its definition into consideration. But why to have such a "feature" at all? Maybe something would happen when transposing more complex piece with constructs I am not aware?

Answer 1

You're right. There's no effect when transposing by a perfect unison. But it does make sense to have the option of transposing by an augmented unison (eg. Ab major to A Major) or a diminished unison (eg. B major to Bb Major). I guess the perfect unison option is simply there because you have perfect, diminished and augmented transpose options for each interval.

Just to be sure, I tried transposing a few passages by a perfect unison using Sibelius 7 (I don't have MuseScore), and yes, the notes are unaffected, but weirdly, the spacing changes…

EDIT: okay, I have spotted one way you can usefully use the transpose by perfect unison function. The transpose function in Sibelius has a "use double sharps/flats" checkbox. You could uncheck this box and transpose by a perfect unison to re-notate the music without double sharps/flats. This is useful; sometimes music is easier to read with more accidentals, but fewer double sharps/flats, even if this is less theoretically "correct". (Eg. G#, G natural, G#, G natural). Having said this, there may be a plug-in for dealing with this anyway.