The abbey of Holy Trinity, Caen, was founded by Mathilda of Flanders, Duchess of Normandy and Queen of England, in June 1066. The abbesses of Holy Trinity are the focus of this study, especially their judicial role and their power to imprison. These rarely discussed aspects of women’s authority are revealed in Manchester, John Rylands Library, GB 133 BMC/66. Produced in 1292 at the meeting of the Exchequer at Rouen, the modest parchment reveals the existence of a prison in Ouistreham, France, under the authority of the abbesses of Holy Trinity. This article engages heretofore unexamined elements of female abbatial authority, jurisdiction and the mechanisms of justice. The preservation of BMC/66 also reflects the documentary imperatives of the women who governed Holy Trinity and fits into a broader context of memory and documentary culture.