Peter N. Schubert in "Hidden Forms in Palestrina's `First Book of Four-Voice Motets'" (Journal of the American Musicological Society, 2007) defines significant blocks of vertical relationships in imitative and non-imitative duos in the thirty-six motets of Palestrina's Motectus festorum totius anni cum communi sanctorum, published in 1564. Schubert describes these blocks of vertical relationships that proceed from duos as modules and organizes them according to categories of construction and function. Palestrina's parody Mass, O Rex glóriæ, reveals the same duos and modules that Schubert discovers in Palestrina's motet of the same name. Palestrina transfers these duos and modules from the motet into the parody Mass, using them as building blocks for points of imitation. The duos, modules, and their motives appear in all but a few places, and are in some cases prominent throughout movements of the Mass, such as the Kyrie. Palestrina manipulates and elaborates these duos and modules according to the character and text of each movement. He borrows them consistently in their original order, which he changes only for reasons of textual meaning or verbal similarity. The module approach to recurring vertical combinations, although a recent application, is valuable for recognizing and treating systematically the duo relationships and their elaboration that are described by late-Renaissance theorists, especially Fray Tomas de Sancte Maria. The identification and analytical interpretation of duos and modules in Palestrina's motet O Rex glóriæ and the parody Mass based on it yields insights not only into his compositional decisions as he adapts material from the motet for its new setting, but also into the potential value of modules as the basis for an analytical approach to the sacred vocal polyphony of the sixteenth century.