During the Middle Ages, written music was most often kept in books separate from the books used for the Mass or Divine Office. The Mass is the Latin liturgical rite in which the Eucharist (bread and wine) is consecrated. The Divine Office is the set of prayers recited at the canonical hours by the clergy. The canonical hours are Matins, Lauds, Prime, Terce, Sext, None, Vespers and Compline. Matins is separated into three parts called nocturns. Music for the Mass was contained in a gradual; music for the Divine Office was contained in an antiphonary (also known as an antiphonal, antiphonale, or antiphoner). “Gradual” is also the term for a portion of the Mass: this term receives its name from the practice of the choir singing from the steps at that point in the service. “Antiphonary” receives its name from antiphons, which are liturgical chants with prose texts that precede and conclude psalms.
Most antiphonaries and graduals were enormous: they had to be seen from a distance by the entire choir. Because they were community books, their decoration and illumination reflected the wealth of the community. Large choir books were expensive to produce: each leaf (which consists of two pages) is an animal skin. One book could have up to 150 leaves.