What is the “Ordinary” of the Mass?

A few years ago—during the Sacred Music Symposium—I suggested to the audience that the “Our Father” and the “Alleluia” should be considered part of the Ordinary of the Mass (for the Ordinary Form). In my own settings, such as Mass of the English Martyrs, I treated the “Our Father” accordingly—and here in Los Angeles, we always treat the “Alleluia” that way. When it comes to the Extraordinary Form, there is a case to be made that “Ite, Missa Est” could be part of the Ordinary of the Mass. Composers of a certain generation—such as Philippe Verdelot (d. 1540) and Father Cristóbal de Morales (d. 1553)—polyphonically set the “Ite, Missa Est.”