INTROIT: The name given in the Latin Church to the anthem at the beginning of the communion service. It usually consists of an antiphon, a verse (or more) from a psalm or other portion of Scripture, and the Gloria Patri (see LITURGICS, III., § 2). It differs considerably in the different rites in name, contents, and the time of its performance. Numerous forms exist, the Pian Missal alone containing 159. The origin is debated, some ascribing it to Pope Celestine (423 A.D.; cf. Liber pontificalis, ed. Mommsen in MGH, Gest. pont. Rom., i. 94, 1898), and others to Gregory the Great.