Everything I know about 16th-Century Italian I learned from 20th-Century Spanish. I hope that someone here is able and willing to check my guesses about this.
The following text seems to follow an introduction to mensural rests, and it surrounds a figure of a single bar line and a double bar line on an otherwise empty staff. The book often uses bar lines in a way that looks semantically modern, though it also contains sample compositions without bar lines. > Gli Ecclesiastici etiandio pongono le Pause ne i loro canti, no gia per > ornamento, ma per necessità. . . . > Il che fa dibisogno, che li Compositori etiandio auertitenza delle parole si > oda, & intenda interamente: percioche facendo in cotal modo, allora si potrà > dire, che le Pause siano state poste nelle parti della cantilena con qualche > proposito, & non a caso. Ne si debbeno porre per alcun modo, auanti che sia > finita la sentenza, cioè nel mezo della Clausula: conciosia che colui, che le > ponesse a cotal modo, dimostrarebbe veramente essere vna pecora, vn goffo, & > vno ignorante. Però adunque il Musico si sforzerà di non cascare in simili > errori; accioche non dia alli dotti mala opinione di se, il che molto si > debbe prezzare, & preporre ad ogn'altra cosa. This is what I've drawn out: When notating a chant, anyone who places full-height single or double lines anywhere but the end of a phrase is a sheep, a klutz, and an ignoramus. My main doubt is whether this restriction is limited to church music. I hope to understand whether to regard the bar lines in the rest of the book as a normal part of the notation that this author is teaching, or rather as a learning aid that he expects the reader not to carry over into his work. More context might be required. The book is available at IMSLP. See page 212. https://imslp.org/wiki/Le_Istitutioni_Harmoniche_(Zarlino,_Gioseffo) Thanks and regards, — Dan