«APERI DOMINE».—Prayer with which the divine office begins. According to the admonition of Eccl. (18:23), the soul must prepare itself for prayer; for this reason, in the Middle Ages the practice arose of first visiting the altars, or reciting psalms, etc. In some monasteries they began with the prayer «A. D.», which the priest said upon approaching the altar (sec. 2), or also in the canon of the Mass after the Preface, while the choir sang the Sanctus, and finally before beginning Matins. S. Pio V recommended it to all the clergy and placed it at the beginning of the divine office.
BIBL.: H. Mahieu, Probatio charitatis, Bruges 1914, no. 159; C. Callewaert, in Collationes Brugenses, 22 (1922), p. 269. Filippo Oppenheim