The origin and development of Western plainchant, and of the genres of liturgical book in which it is recorded, have occupied Michel Huglo throughout his long career, which has taken him to libraries in every corner of Europe and the United States. This volume, the first in a set of four to appear in the Variorum series, brings together analyses of manuscripts dating from the 9th to the 13th century, including Huglo’s pathbreaking studies of the antiphoner of Compiegne, the first troper-prosers, and of alleluia lists as clues to place of origin. The consequences of the Treaty of Verdun (843) for the diffusion of the plainchant repertory, research in medieval musicology in the 20th century, the utility of codicology for musicological manuscript studies, and the critical edition of the Gregorian antiphoner are addressed in other studies included here. Les origines et le developpement du plain-chant en Occident et l'etude des genres de livres liturgiques qui le contiennent ont occupe Michel Huglo durant sa longue carriere et l'ont conduit A visiter des bibliotheques partout en Europe et aux Etats-Unis. Ce volume, le premier d'une serie de quatre dans la collection Variorum, comprend des analyses de manuscrits du neuvieme au treizieme siecle, notamment des etudes novatrices relanAant les recherches sur l'antiphonaire de Compiegne, les premiers tropaires-prosaires et les listes d'alleluias comme moyen d'identification des manuscrits de chant. Les consequences du traite de Verdun (843) pour la diffusion du repertoire de plain-chant, les recherches en musicologie medievale au XXe siecle, l'application des methodes de la codicologie A l'etude des manuscrits notes, et l'edition critique de l'Antiphonaire gregorien forment les sujets d'autres etudes reunies dans ce volume.