This is a huge issue in musicology. Last year I was in the process of setting a psalm text (in Latin) to one of the Palestrina falso bordone, and asked a friend who had done it before what the rules were.

As it turned out, he had consulted with Dr. Stephen Plank of the Oberlin Conservatory on this, and was told that essentially there aren't any hard and fast rules that can be discerned, but that the place to start is to identify the voice that carries the original psalm tone (or its approximate profile) and assign the text accordingly. Then move on to the next voice with the least amount of extra notes and assign the syllables so that they either concur or occur after they appear in the chant melody. (In other words, from what I've been told, a syllable of text may not appear in any of the other voices until it appears in the cantus, although they may occur simultaneously). Continue applying this technique to each subsequently more complex voice. What you'll find in really complex falsos is that there will be some voices that will have places where the syllables will fall on awkward passages of notes, and you have to fudge them until they make "sense" without violating the one rule I've described above.

(By the way, it probably goes without saying, but I'll say it anyway, that only every other verse of text is set. The intervening verses are sung to the original psalm tone. Sometimes this means transposing the music of the falso so that the other voices aren't too high or too low, while keeping the psalm tone itself at a reasonable pitch range.)

Clear as mud? Godspeed.

Thank you David Andrew . and Jeff Ostrowski, your pages are very helpful .