As the first printed chant book to align with the reformed Roman Rite, the Antiphonarium Romanum (Antwerp: Plantin, 1571–73) marked a turning point in the creation of liturgical editions. Documents from the Plantin-Moretus Archief provide unprecedented information concerning the antiphoner’s commission, revision, and production. Letters from printer Christopher Plantin to his patrons, Cardinal Antoine Perrenot de Granvelle and Bishop Gilbert d’Oignies, reveal a power struggle over the plainchant: whether to use Granvelle’s Roman exemplar, or d’Oignies’s Low Countries manuscript. Plantin finally chose to follow the instructions of his most influential patron, Granvelle. Analyses of the Antiphonarium’s musical preface and the responsory Felix namque demonstrate that the volume transmitted a prescriptively notated chant tradition revised according to contemporary theoretical precepts. The Commune Sanctorum underwent further modification in its second edition (1574) to bring it visually and musically closer to the rest of the antiphoner, as evidenced in the responsory Suscipe verbum. The Antiphonarium had a lasting influence in the Southern Netherlands, both through its adoption by important religious institutions and its use as a copytext by other printers. Its plainchant provided an audible signal of allegiance to sacred and secular authorities and embrace of the Catholic Reformation.