«APERI DOMINE». — Prayer with which the divine office is begun. According to the admonition of Ecclesiasticus (18:23), the soul must prepare itself for prayer; for this reason in the Middle Ages the custom arose of first visiting the altars or reciting psalms, etc. In some monasteries the prayer «A. D.» was begun, which the priest said as he approached the altar (10th century) or even in the canon of the Mass after the Preface, while the choir sang the Sanctus, and finally before beginning the morning. St. Pius V recommended it to all the clergy and placed it at the beginning of the divine office.
BIBL.: H. Mahieu, *Probatio charitatis*, Bruges 1914, n. 159; C. Callewaert, in *Collationes Bruggenses*, 22 (1922), p. 269. Filippo Oppenheim