The Abbey of St. Gertrude in Nivelles was an important foundation in Belgium from the seventh century onward. The double chapter of women and men, ruled by a noble abbess, was a powerful ecclesiastical and political force. This manuscript, recently discovered, is essentially the only medieval source for the practices of Nivelles. It details the complex liturgical practices of this double house in the thirteenth century. The presence of women and men in the same institution and participating often in the same liturgical actions, is rare in practice, and rarer still in descriptions of such detail. We are provided with information about processional paths, local topography, the layout of the church of St. Gertrude, the adjacent St. Paul’s and St. Mary’s, and the rest of the precincts of the institution. The manuscript also includes a series of capitular and historical documents which detail the relationships, and the stress, between the Abbess and the Chapter, and illuminate aspects of local and regional history, including such details as the length of the portion of salmon that the Abbess is required to provide on Maundy Thursday. The manuscript is accompanied by a detailed kalendar. The edition presented here provides a careful study of the manuscript and its contents, a detailed introduction to the medieval liturgy of Nivelles, and comprehensive indices.