Texting in late fifteenth-century music has long challenged musicologists and editors, who often use sixteenth-century theories to explain earlier repertories. This article examines texting in polyphonic mass ordinary settings. It analyzes the role played by composer, scribe, and performer, and examines how texting differed depending on the repertory. Taking Josquin’s Missa L’homme armé super voces musicales as a case study, this article investigates one source in particular, Vatican City, Biblioteca Apostolica Vaticana, Cappella Sistina 197, which contains an almost unique instance of coeval additions and corrections to the original scribal texting. This manuscript allows us to assess the priorities of the emendator(s) in modifying the original text underlay, and thus provides some fifteenth-century texting principles for modern editors.