From volume 3 of the work.

2114515A Dictionary of Music and Musicians — PlicaWilliam Smyth Rockstro

PLICA (literally, a Fold, or Plait). A character, mentioned by Franco of Cologne, Joannes de Muris, and other early writers, whose accounts of it are not always very easily reconciled to each other. Franco describes four kinds: (1) the 'Plica longa ascendens,' formed by the addition, to a square note, of two ascending tails, of which that on the right hand is longer than that on the left; (2) the 'Plica longa descendens,' the tails of which are drawn downwards, that on the right being, as before, longer than that on the left; (3) the 'Plica brevis ascendens,' in which the longer of the ascending tails is placed on the left side; and (4) the 'Plica brevis descendens,' in which the same arrangement obtains with the two descending tails.

Joannes de Muris describes the Plica as a sign of augmentation, similar in effect to the Point. Franco tells us that it may be added at will to the Long, or the Breve; but to the Semibreve only when it appears in Ligature. Some other writers apply the term 'Plica' to the tail of a Large, or Long. The Descending Plica is sometimes identified with the Cephalicus, which represents a group of three notes, whereof the second is the highest. [See Notation, vol. ii. pp. 467, 468.]

[ W. S. R. ]