If Dom Mocquereau was correct in his interpretation (and I believe he was), it doesn't matter where the quilisma is actually positioned on the staff, only the position of the preceding and following notes. Perhaps the placement in the Riesenkodex acknowledges that the portamento begins on the preceding note.[T]he quilisma may be looked upon as a kind of ascending portamento. This is a plausible interpretation, for it agrees with the data furnished by the MSS. As a matter of fact an ascending portamento invariably requires the lower note to be sustained, for the voice rests on it before gliding smoothly upwards, and this is just what the MSS indicate. These scanty references in mediaeval writers will not perhaps content all our readers, and those who wish are free to treat the quilisma as an ordinary note, light, and without any portamento. This makes an easier rendering, which is a point worthy of consideration. Even in this case however, the lengthening of the notes preceding the quilisma must be maintained, for this is an ascertained fact. (Mocquereau: Le Nombre Musical Gregorien/A Study of Gregorian Musical Rhythm, p. 419)
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