INTROITO. — This is what the chant performed in the Roman liturgy is called while the celebrant, during the solemn Mass, proceeds to the altar. Like the invitatory in the Office, the I. interprets and expresses the sentiments proper to the mystery or feast of the day.
The Gelasian Sacramentary of the eighth century calls it «antiphona ad introitum» (cf. also the Ordines romani I [n. 8], II [n. 3], III [n. 8], and the Ordo Ioannis Archicantoris); the Hadrianic Gregorian Sacramentary, on the other hand, gives it the name «introitus», and the Ordo rom. V (eleventh century) that of «invitatorium».
The Lib. Pont. (I, p. 230) has Celestine I (422–32) institute it, but the passage is unclear, and it is not known whether it refers to the i., or rather to the singing of the psalms of the vigils before the Sacrifice. Consequently, Probst attributes its origin to Gelasius I (492–96), while Batiffol dates its introduction to the sixth century.
The I. originated as a chant of the schola, not of the assembly. This explains the richness of the melody, quite different from the recitative of chants intended for the mass of the faithful.
The Ordo rom. I (n. 40 ff.: M. Andrieu, Les Ordines romani, II, Leuven 1948, p. 80) describes the singing of the i. as follows: the singers arrange themselves in two rows before the altar. When the Pontiff leaves the secretarium, the parafonist (prior scholae) intones the antiphon, which is performed in its entirety by the first choir and then repeated by the second. The first choir then sings the verses of the psalm, alternating with the antiphon-refrain, which is always sung by the second choir. When the Pontiff reaches the altar, the Gloria is sung, followed by the versus ad repetendum (a characteristic verse, which had determined the choice of the psalm), sung by the first choir, while the second concludes with the repetition of the antiphon. This solemn performance had to be simplified in the paroeciae (rural churches) and in monastic oratories, which had few clergy. The psalm was reduced to one or two verses. Then the antiphon after the verses too was repeated only halfway (at Cluny, eleventh century: PL 149, 653), until it disappeared altogether, leading to the present arrangement sanctioned by the Missal of Pius V (1570). The abbreviation Ps(almus), instead of V(ersus), still placed before the verse recalls the primitive form of the i. Pius X (Graduale Romanum, Rome 1907) restored the practice of singing the I. «accendente sacerdote ad altare», and the S. Congr. of Rites recently granted permission to sing it «more antiquo» (cf. Ephém. lit., 62 [1948], pp. 248–55).
The principal source of the I. is the psalms. Others derive from Scripture readings (the oldest) or from free composition, e.g., Salve sancta Parem by Sedulius; Gaudeamus by St. Gregory (d. 604); or from other sources, such as Accipite iucunditatem for the Tuesday after Easter, and Requiem of the funeral liturgy, which Morin regarded as among the oldest because they were taken from the Book of Ezra, excluded from the canonical books in the fourth century. Centonization is not uncommon, as in the I. Resurrexi of Easter. In the Middle Ages the I. too were adorned with tropes (v.), but all of these disappeared.