Final answer:
The compositional settings of the Mass Ordinary are musical units, not liturgical units. They can be rearranged or combined in different ways, while the liturgical units have a fixed order and function within the Catholic Mass.
Step-by-step explanation:
In the Renaissance period, the principal liturgical forms were masses and motets. Composers of sacred music began to adopt secular forms like the madrigal, for their own designs. While the Mass Ordinary consists of five main sections, called musical units, it does not necessarily align with the liturgical units of the Mass Ordinary.
The musical units were composed separately and can be rearranged or combined in different ways, while the liturgical units of the Mass Ordinary have a fixed order and function within the Catholic Mass.