This article aims to provide insight into the nature, distribution and function of certain Middle Dutch translations of Latin hymns and sequences that originated in the circles of the Devotio Moderna. Unlike the vernacular versions in (most) Middle Dutch lay breviaries, which were used as texts for prayer in the context of private devotion, the translations in what I refer to as “vernacular mass and office books” functioned as subtitles to the Latin liturgy. This type of book was primarily intended for canonesses regular, religious women who had to attend the liturgical services of the Divine Office and of Mass, but had not (fully) mastered Latin. Mass and office books originated in the eastern part of the northern Netherlands, whereas the lay breviaries were intended for tertiaries from the western side of the diocese of Utrecht. These women, who followed the rule of the Third Order of St Francis, were not obliged to attend the liturgical services. In order to illustrate the nature and function of the mass and office books, the article focuses on the books of the canonesses regular of St Agnes's in Maaseik.