Practica Musicae utriusque cantus, excellentis Franchini Gaffori Laudensis quatuor libris modulatissima

...note of the ligature is described with a square body under the penultimate as here:
Musical notation showing a ligature with a square note body.
Or directly above the penultimate as here:
Musical notation showing a ligature with a square note body.

Or indirectly above the penultimate, that is, in a straight degree, with a square body and a stem descending on the right side in this manner:
Musical notation showing a ligature with a square note body and a descending tail on the right.

Therefore, perfection is the appropriate figuration for the final notes of ligatures. Those who dissent from these positions depart from the authority of antiquity. Where it happens that one who describes the last note of a ligature with an oblique body as here:
into error is led. However, only two notes are conceded in an oblique body in this manner:
But the oblique body ascending as here:
Musical notation showing oblique ligatures.
is abhorred by use.

But for the middle notes of ligatures, which are joined between the extremes, no diversity of figuration is ascribed; for they are figured like simple square ones, or oblique in this manner:
Musical notation showing a series of square and oblique notes in a ligature.
Furthermore, every ligature, however many notes it comprises, extracts a single syllable to be pronounced. Therefore, all notes of this kind of musical progression, even if they are described with different figurations, ought to be pronounced with an equal measure of time. A mediocre note is one that takes the form neither of simple nor composite figures; but in its simplicity, it is compared to an oblique body by a certain similarity, and it is not described alone, but two or more are frequently noted in the form of a measurable semibreve, in this manner:
Musical notation showing a series of diamond-shaped notes.

They are, however, equal to the others in pronunciation and measure of time, although some measure them twice as strictly as the others, which we have decided to concede not by reason, but by the will of the cantor. For many have wished to follow the pronunciation and measure of plainchant notes by their own authority. Hence, the saying of Persius is not inappropriate: "Each has his own will; one does not live by a single wish." There are also those who do not—Persius: describe and measure notes of this plainchant such as these equally with figures of measurable consideration, such as longs, breves, and semibreves, as is evident in the Cardinal's Symbol The Apostles' Creed and in some proses and hymns, which the French, most notably, pursue very celebratedly for the ornamentation of modulations by conceiving the pronunciation itself through that very diversity.