Prefatory Prayer from Catholic Encyclopedia (1913) The first verse of the sixty-ninth Psalm

These words form the introductory prayer to every Hour of the Roman, monastic, and Ambrosian Breviaries, except during the last three days of Holy Week, and in the Office of the Dead.

While they are said, or sung, all present sign themselves with the sign of the cross. Tradition says that St. Benedict introduced this custom into the monastic Office and that St. Gregory I extended it to all the Roman churches; Cassian (Coll., X, 10), however, says that from the earliest Christian times the monks used this introduction very often, probably outside of the liturgical prayers.

In placing this supplication at the beginning of every Hour the Church implores the assistance of God against distractions in prayer. In the Roman Rite the “Deus in adjutorium” is preceded in Matins by the “Domine labia mea aperies”, whilst in the monastic Breviary the order is reversed.