Every rite ends its liturgy with a short prayer or two and a blessing before the dismissal
Postcommunion.—The Communion act finishes the essential Eucharistic service. Justin Martyr (I Apol., lxv-lxvi) adds nothing after describing the Communion. However, it was natural that the people should not be dismissed without a final prayer of thanksgiving and of petition, so every rite ends its liturgy with a short prayer or two and a blessing before the dismissal. The earliest complete liturgy extant, that of the “Apostolic Constitutions“, VIII, contains two such prayers,—a thanksgiving (XV, ii-vi), and a blessing (XV, vii-ix). A significant resemblance between the Roman Rite and that of the “Apostolic Constitutions” is that at Rome, too, there were formerly at every Mass two prayers of the same nature. In the “Leonine Sacramentary” they have no title; but their character is obvious. As examples, those for the summer ember days may serve (ed. Feltoe, p. 51, “In jejunio”), the first Gratias tibi referimus, the second Oculis tuts miserationis intende. The Gelasian Sacramentary calls the first postcommunio, the second ad populum. In both sacramentaries these two prayers form part of the normal Mass said throughout the year, though not every Mass has both; the prayers “ad populum” in the later book are comparatively rare. They also begin to change their character. The formerly constant terms tuere, protege etc. are rarer; many are ordinary collects with no pronounced idea of prayers for blessing and protection. In the “Gregorian Sacramentary” the second prayer, now called Super populum, occurs almost only from Septuagesima to Easter; the first, Ad complendum, continues throughout the year, but both have lost much of their original character. The Ad complendum prayer (Postcommunion) has become a collect formed on the model of the collect at the beginning of Mass, though generally it keeps some allusion to the Communion just received. That is still the state of these prayers after the Communion. The second, Oratio super populum, is said only in ferial Masses in Lent. This restriction apparently results from the shortening of the Mass (which explains many omissions and abbreviations) and the tendency of Lent to keep longer forms. The Mass was shortened for practical purposes except (in many cases) during Lent, which keeps the long preces in the Office omitted at other times, sometimes more than two lessons at Mass, and soon. The medieval commentators (Amalarius, “De divinis officiis”, III, xxvii; Durandus, “Rationale“, VI, xxviii; Honorius of Autun, “Gemma anima'”, lix) explain this mystically; Honorius thinks the prayer to be a substitute for the Eastern blessed bread (Greek: antidoron). The Oratio super populum is now always the prayer at vespers on the same day. It has been suggested that its use at Mass in Lent may be a remnant of a custom, now kept only on Holy Saturday, of singing vespers at the end of Mass (Gihr, op. cit., 711). There remains the first prayer, called Ad complendum in the “Gregorian Sacramentary”. Its name was uncertain through the Middle Ages. Durandus (op. cit., IV, lvii) calls it merely Oratio novissima, using the name Postcommunio for the Communion antiphon. The first “Roman Ordo” calls the prayer Oratio ad corrtplendum (xxi); Rupert of Deutz calls it Ad complendum (De divinis officiis, II, xix). But others give it the name it had already in the Gelasian book, Postcommunio (Sicardus, “Mitrale”, III, viii); so also many medieval missals (e.g., the Sarum). This is now its official name in the Roman Rite. The Postcommunion has lost much of its original character as a thanksgiving-prayer and has absorbed the idea of the old Oratio ad populum. It is now always a petition, though the note of thanksgiving is often included (e.g. in the Mass Statuit, for a confessor pontiff). It has been affected by the Collect on which it is modeled, though there is generally an allusion to the Communion.
Every Postcommunion (and secret) corresponds to a collect. These are the three fundamental prayers of any given Proper Mass. The Postcommunion is said or chanted exactly like the Collect. First comes that of the Mass celebrated; then, if other Masses are commemorated, their Postcommunions follow in the same order and with the same final conclusion as the collects. After the Communion, when the celebrant has arranged the chalice, he goes to the epistle side and reads the Communion antiphon. He then comes to the middle and says or sings Dominus Vobiscum (in the early Middle Ages he did not turn to the people this time—”Ordo Rom.”, I, n. 21), goes back to the Epistle side, and says or sings one or more Postcommunions, exactly as the collects. At ferial Masses in Lent the Oratio super populum follows the last Postcommunion. The celebrant sings Oremus; the deacon turning towards the people chants: Humiliate capita vestra Deo, on do with the cadence la, do, si, si, do for the last five syllables. Meanwhile, everyone, including the celebrant, bows the head. The deacon turns towards the altar and the celebrant chants the prayer appointed in the Mass. At low Mass the celebrant himself says: humiliate capita vestra Deo and does not turn towards the people. The deacon’s exclamation apparently was introduced when this prayer became a specialty of Lent. Durandus mentions it (VI, xxviii).