The Tongeren collection, belonging to one of the oldest churches of Onze-Lieve-Vrouwekerk in de Nederlanden (Our Lady in the Low Countries), forms an exceptional collection of Gregorian manuscripts. Primarily, the manuscripts contain numerous chants from the local repertoire, for example, offices for Saints Remaclus, Maternus and Servatius. Additionally, this collection documents the significant liturgical reform of Radulphus de Rivo, inspired by the Modern Devotion and the Windesheim Congregation, and is an important witness to the Liège rite in Flanders. The ‘Liber ordinarius’ is one of the most remarkable sources for this.