When one realises that thousands of liturgical chants were transmitted over more than three centuries in which there were no accurate means of melodic notation, the core repertoire of Gregorian plainchant has been transmitted with an astonishing degree of uniformity. On the other hand, these chants are reflections of a living liturgy, and thus reflections of their particular surroundings: any monastic or collegial environment could develop its own conventions in relation to what only seems to be a uniform repertoire at first sight. Although relationships between specific manuscripts and their associated variants of Gregorian plainchant certainly exist, how the different variants of chants developed continues to be a highly debated topic. Moreover, how these different developments were valued has varied widely. This study sets out to map the melodic variation occurring in northern Low Countries chant manuscripts from the oldest known notations to those of the sixteenth century, and to explore their relationships and attributions through this variation.