Liturgy may be defined as the practice the practice of organized, formal worship. the ensemble of texts, music (books and instruments), movements, vestments, vessels, and even the buildings and the interaction of all of these used by a group of people to worship God. Liturgy encompasses two main types of service: the Mass and the Divine Office.

The Mass derives its name from its closing formula, “Ite, missa est” meaning, “Go, it is the dismissal.” It consists of two parts: the first contains prayers and readings from the Bible, such as were customary to the Jewish services from which the “liturgy of the word” derives. The second part contains the consecration of the bread and wine as the Body and Blood of Christ, according to the mandate in the Gospels, “This is my body… This is my blood… do this as a memorial of me” (Mark 14:22-25; 1 Cor. 11:23-25). The two parts were already fused into a single service by the middle of the second century, as shown by the writings of Justin Martyr, although the core of the second and specifically Christian part of the Mass, called the Canon, only achieved its completed form during the sixth century. The Canon is the central section of the Mass, called by this name to indicate its “canonical” or fixed nature, in that its arrangement of prayers and rites are not subject to variations due to the seasons of the liturgical year, or to the respective rankings of any coinciding feasts. These prayers of consecration of the bread and wine are said by the priest at every Mass.

An Antiphonal (or antiphoner or antiphonary) contains the sung parts of the Divine Office that are specific to Sundays, feasts, and the Common of Saints. The chants found in an antiphonal include the antiphons sung with the psalms and canticles, the responsories of Matins and Vespers, and sometimes the hymns. Antiphonals were divided into two sections, the first encompassing the Proper of Time or the temporale (beginning with the first Sunday of Advent), and the second organized around the Proper of Saints or the sanctorale.

A Gradual contains the sung portions of the Mass, particularly the chants for feasts, Sundays, and the common of saints. In each Mass several chants were “proper” to the occasion, including the introit, gradual, sequence, offertory, and communion. The ordinary chants were those sung at every Mass, including the Kyrie, Gloria Credo, Sanctus, Agnus Dei, and Ite Missa Est.

A Processional contains the chants sung during the processions on certain important feasts of the church year, including Christmas, the Purification of the Virgin Mary, or Candlemas (February 2), Ash Wednesday, the Adoration of the Cross on Good Friday, the major litanies (April 25) and Rogations (during three days before the feast of the Ascension). Processionals began as small sections of chants included within chant books in the tenth and eleventh centuries, and gradually developed into separate books, while retaining their small size for portability.