Conceptions of the compositional process changed fundamentally over the course of the Renaissance. In the medieval era, composers were often able to layer voices one on top of the other, as in organum, clausulae, and isorhythmic motets. Cantus firmus compositions continued this tradition of successive composition to some extent. But the more homogeneous texture evident in a work like Josquin’s Ave Maria . . . virgo serena suggests a different approach to writing. This kind of texture, based on interlocking points of imitation, required composers to work out the implications of any given idea for all the voices more or less at once. Here, the theorist Pietro Aron (1489–1545) describes this process of simultaneous conception in an account first published in 1529.

"Many composers were of the opinion that the soprano should be composed first, then the tenor, and after the tenor the bass. This happened because they lacked the order and understanding of what was necessary to compose the alto. Thus they had many awkward places in their compositions because they had to insert unisons, pauses, and ascending and descending leaps that were difficult for the singer or performer, so that those works had little sweetness and harmony. For in composing the soprano first and then the tenor, once the tenor was made there was sometimes no room for the bass, and once the bass was made, there was no place for many notes in the alto. Therefore, in considering only part by part, that is when the tenor is being composed, if you pay attention only to harmonizing this tenor [with the soprano], and the same with the bass, it is inevitable that each part will suffer where they come together. Therefore the modern composers had a better idea, which is apparent from their compositions in four, five, six, and more voices, in which each part has a comfortable, easy and agreeable place, because they take all the parts into consideration at once and not as described above. And if you prefer to compose the soprano, tenor, or bass first, you are free to follow that method and rule, as some at present do, who often begin with the bass, sometimes with the tenor, and sometimes with the alto. But because this will be awkward and uncomfortable for you at first, you will begin part by part; nevertheless, once you have gained some experience, you will follow the order and method described before."

Source: Pietro Aron, Toscanello in musica (1529 ed.), Book II, Chapter 16, trans. Bonnie Blackburn in "On Compositional Process in the Fifteenth Century," Journal of the American Musicological Society 40 (1987): 215.