This page was extracted with permission from an email from Margo Schulter.
A few composers of the 16th and early 17th centuries, most notably Nicola Vicentino (1511-1576), use an interval known as the "enharmonic diesis," equal more or less to 1/5-tone on a meantone keyboard with pure major thirds. This is equal to the distance between G# and Ab on a meantone keyboard -- most comprehensible, of course, on a keyboard instrument with more than 12 notes which has both these tones to compare.
Anyway, the few compositions and portions of compositions that have come down to us in this kind of style divide each tone into five or more less equal parts -- actually very slightly unequal in meantone with pure major thirds -- like this:C C* C# Db Db* D
Here I use an ASCII equivalent of Vicentino's notation: where I have an asterisk, Vicentino likewise puts a dot above the note to show that it is raised by a diesis or "fifthtone," as I call it.
Thus C* is about halfway between C and C#, while Db* is halfway between Db and D. Vicentino's own archicembalo or "superharpsichord" has all these notes, and divides the octave into 31 nearly equal fifthtones.
Anyway, I've been using a 24-note keyboard instrument with a subset of this system -- actually a synthesizer and two regular 12-note keyboards tuned in meantone a diesis apart. The question occurred to me: "How can I show that a note is raised by a diesis with abc2ps?"
Then, reading the documentation, I realized that the "staccato" option could put a dot above or below the note, a very close approximation of Vicentino's own notation. The one difference is that sometimes the dot will appear below the note rather than above, but I consider this a very small point which shouldn't cause any problem with readability.
Incidentally, Vicentino suggests that performing trying this kind of music use his keyboard as a guide to tuning; both now and then, performers may have problems finding these microtonal intervals without such an instrument.