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...