Catholic Encyclopedia (1913)/Orate Fratres
The exhortation ("Pray brethren that my sacrifice and yours be acceptable to God the Father almighty") addressed by the celebrant to the people before the Secrets in the Roman Mass. It is answered: "May the Lord receive the sacrifice from thy hands to the praise and glory of his name, and for our benefit also and for that of all his holy Church." The celebrant adds: "Amen". The form is merely an expansion of the usual Oremus before any prayer. It is a medieval amplification. The Jacobite rite has an almost identical form before the Anaphora (Brightman, "Eastern Liturgies", Oxford, 1896, 83); the Nestorian celebrant says: "My brethren, pray for me" (ib., 274). Such invitations, often made by the deacon, are common in the Eastern rites. The Gallican rite had a similar one (Duchesne, "Christian Worship", London, 1904, 109). The Mozarabic invitation at this place is: "Help me brethren by your prayers and pray to God for me" (P.L. LXXXV, 537). The medieval derived rites had similar formulæ (e. g. "Missale Sarum", Burntisland, 1861-3, 596). Many of the old Roman Secrets (really Offertory prayers) contain the same ideas. Durandus knows the Orate Fratres in a slightly different form ("Rationale", IV, 32). A proof that it is not an integral part of the old Roman Mass is that it is always said, not sung, aloud (as also are the prayers at the foot of the altar, the last Gospel etc.). The celebrant after the "Suscipe Sancta Trinitas" kisses the altar, turns to the people and says: Orate fratres, extending and joining his hands. Turning back he finishes the sentence inaudibly. At high Mass the deacon or subdeacon, at low Mass the server, answers. The rubric of the Missal is: "The server or people around answer, if not the priest himself." In this last case he naturally changes the word tuis to meis.
GIHR, The Holy Sacrifice of the Mass (3rd ed., St. Louis, 1908), 547-50.