A gradual is the response and versicle to the Epistle reading that constitutes one part of the Mass. The name derives from the practice of singing the gradual on the steps of the raised pulpit. More commonly, however, the term refers to the principal choir book used in the Mass. Arranged according to the liturgical year (with Temporale, Sanctorale, and Common of Saints), a gradual contains (in addition to the graduals themselves) introits, tracts, alleluias, offertories, and communions. The introits - the first sung elements of the Mass - were often introduced by historiated initials (Ad te levavi, the introit for the first Sunday in Advent, being the most elaborate). For low Mass, the contents of the gradual were included in the Missal and performed by the celebrant rather than the choir.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).