I have not encountered an author who has written a treatise on the tons sung in the church (which the organist should know) who has been able to clear up the question of their ranges for those who wish to learn. That is why I have decided to write about them: how I have learned them, and how they are observed in the plainchant sung in churches; how organists should play them and end them for the convenience of the choir; and afterward, how fugues and subjects should be treated.

First of all, it should be known that those who composed the antiphons and the chants of psalmody employed only the diatonic [genus], which is one of the three genera of music, as you may read above. As a result, they used only the naturals of the keyboard and did not make use of the feintes or sharps, so as not to present so much difficulty to those who wished to learn to sing the plainchant. However, they did observe the B♭, and made it known that anything sung above la should be sung as fa, which is quite wrong if it is not marked. Anyone who sings music or plainchant is obliged to sing only what he sees written down. To do otherwise would be to compose, not to sing, as I could prove through numerous examples.