The Birgittine Order, founded by St Birgitta of Sweden in the 14th century, was organized in double abbeys ideally consisting of 60 sisters and 13 brothers. This had far-reaching consequences for the organisation of their liturgy, which was defined by Birgitta’s authority as expressed in her revelations and her rule. The reasons for the double-abbey solution were both practical and spiritual, to supply the sisters with confessors and priests but also to complement the sisters’ Marian observance with the brother’s diocesan liturgy. Thus the liturgical unity of Birgittine spirituality could be maintained while observing the liturgical year. This paper explores the use at the mother abbey Vadstena in Sweden in the 15th century, focusing on the Friday Sext.