In 1615 Giovanni Maria Trabaci (c. 1675--1647) published the Cento Versi---one non-cantus firmus keyboard versets for use in both Mass and Divine Office---as a book within Il Secondo Libro de Ricercate. Trabaci's music utilized recent developments in liturgical practices, composition, organ building and voicing to reflect the liturgical and musical practices of the early seventeenth century in Naples. The composer created a new kind of organ music free of chant cantus firmi and composed in the eight church tones newly derived from the psalm tones. The versets exhibit forward thinking in liturgical performance while they also continue the long-standing use of the organ in alternatim psalmody. The Cento Versi provided a seventeenth-century keyboard tonary for use in church services. By including these versets with other liturgically appropriate pieces in his book, Trabaci provided a complete resource for the organist that was easy and efficient to use. He also wrote commentaries on the Cento Versi about the theoretical bases for the versets, although he remained silent about their lack of chant quotations and specific liturgical uses. Moreover, Trabaci contrasted the tonal system of the Cento Versi to the modally ordered ricercars of his two keyboard books, but he did not address their differing liturgical applications. The case is made here that both the Cento Versi and the ricercars served liturgical purposes, which is evidenced by their respective collations in cyclic order. The liturgical changes associated with the Tridentine reforms affected the composition of the Cento Versi. The differences between medieval liturgies and changes promulgated by the Council of Trent are the result of precise codifications of liturgical practices (a new development made possible by printing), pruning of the Divine Office; and specific designations of interpolated organ music in the liturgies. In this context, the Cento Versi stand as evidence of leaner liturgies that required flexible, independent musical resources that were easy to use.