Throughout the Middle Ages, the religious women of Nivelles Abbey governed one of the most venerable and powerful ecclesiastical institutions in the Holy Roman Empire, which played a critical role, not only as the center of the cult of St Gertrude, but also as a lynchpin in the power politics of the empire. The recent discovery of the oldest surviving manuscript from the abbey, its Liber ordinarius, thus represents a significant addition to knowledge, not only of Nivelles' liturgy and the development of the cult of its patron saint, but also of the history of female monasticism in the High Middle Ages. In addition to a wealth of detail concerning the abbey's liturgical ceremonies, the Liber ordinarius permits fresh insight into the balance of power in this politically highly competitive region in the thirteenth and early fourteenth centuries. It also sheds light on the history, religious life, and the architectural history of the building, which was badly damaged in WWII. The documents incorporated in the manuscript, most of which were previously unknown and which are edited here for the first time, enhance greatly what is known about the politics of the period as well as the inner workings of the abbey at a time of economic and administrative conflict.