INTROIT. - This is the name given in the Roman liturgy to the chant performed while the celebrant, in the solemn Mass, goes to the altar. Like the invitatory in the Office, the I. interprets and expresses the sentiments proper to the mystery or the feast of the day.
The Gelasian Sacramentary of the 8th 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 Gregorian Sacramentary of Hadrian gives it instead the name "introitus," and the Ordo rom. V (11th century) that of "invitatorium."
The Lib. Pont. (I, p. 230) attributes its institution to Celestine I (422-32), but the passage is not clear and it is not known whether it refers to the I., or rather to the singing of the psalms of the vigil, before the Sacrifice. Hence Probst assigns its paternity to Gelasius I (492-96), and Batiffol places its introduction in the 6th century.
The I. was born as a chant of the schola, not of the assembly. This explains the richness of the melody, quite different from the recitative of the chants destined for the mass of the faithful.
The Ordo rom. I (n. 40 ff.: M. Andrieu, Les Ordines romani, II, Louvain 1948, p. 80) describes the singing of the I. as follows: the singers arrange themselves in two rows before the altar. When the Pontiff comes out of the secretarium, the paraphonist (prior scholae) intones the antiphon, performed in full by the 1st choir, then repeated by the 2nd. Then the 1st sings the verses of the psalm, alternated with the antiphon-refrain, always sung by the 2nd choir. When the Pontiff has reached 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 1st choir, while the 2nd concludes with the repetition of the antiphon. This solemn performance must have been simplified in the paroeciae (rural churches) and in monastic oratories, which had few clergy. The psalm was reduced to one or two verses. Then even the antiphon after the verses was repeated only in half (at Cluny, 11th century: PL 149, 653), until it disappeared entirely, arriving at the current arrangement sanctioned by the Missal of Pius V (1570). The abbreviation Ps(almus) in place of V(ersus), still found before the verse, recalls the primitive form of the I. Pius X (Graduale Romanum, Rome 1907) restored the use of singing the I. "accendente sacerdote ad altare," and the S. Congr. of Rites has recently granted the faculty of singing it "more antiquo" (cf. Ephém. lit., 62 [1948], pp. 248-55).
The principal source of the I. is the psalms. Others derive from the reading (the most ancient) or from free composition, e.g., Salve sancta Parens by Sedulius; Gaudeamus by St. Gregory (d. 604), or from other sources such as: Accipite iucunditatem of the Tuesday after Easter, and Requiem of the funeral liturgy, considered by Morin among the most ancient, because taken from the book of Ezra, excluded from the canonical books in the 4th century. Centonization is not rare, as in the I. Resurrexi of Easter. In the Middle Ages the I. too were adorned with tropes (v.), but all of them disappeared.