Gregorian chant is the central tradition of Western plainchant, a form of monophonic liturgical music within Western Orthodoxy that accompanied the celebration of Mass and other ritual services. It is named after Pope Gregory I, Bishop of Rome from 590 to 604, who is traditionally credited for having ordered the simplification and cataloging of music assigned to specific celebrations in the church calendar. The resulting body of music is the first to be notated in a system ancestral to modern musical notation. In general, the chants were learned by the viva voce method, that is, by following the given example orally, which took many years of experience. Gregorian chant originated in monastic life, in which celebrating the 'Divine Office' eight times a day at the proper hours was upheld according to the Rule of St. Benedict. Singing psalms made up a large part of the life in a monastic community, while a smaller group and soloists sang the chants. In its long history, Gregorian chant has been subjected to many gradual changes and some reforms.