This paper identifies a neglected manuscript, Viterbo, Centro Diocesano di Documentazione (CeDiDo), Capitolare 51 (R), as the extant archetype of the Libri Romani version of the glossary of Placidus. It first demonstrates that R is the parent of the three witnesses to the Libri Romani text used by editors, and it considers the implications of the neglected manuscript for future editions of the text. It then corroborates the importance of R by tracing its travels in humanistic and antiquarian circles in Italy in the fifteenth and sixteenth centuries. This history provides a framework for future research on the textual transmission of the Libri Romani text of Placidus.
