Despite the evident preference of the Consilium for celebration of the liturgy of the Eucharist versus populum, the 1969 Missale Romanum did not require or even mention the practice in its treatment of the various forms of Mass (see General Instruction of the Roman Missal [GIRM], 74-252). On the contrary, then as now the forma typica of the Mass (GIRM 82-126) envisions celebration ad orientem during the liturgy of the Eucharist.
The priest is specifically instructed when to face the people (namely, for the “Pray brethren” [GIRM 107], the Sign of Peace [GIRM 112], the “Behold the Lamb of God” [GIRM 115] and the post-communion prayer [GIRM 122], although the rubric in the sacramentary at this point [n. 142] states that he turns only for the blessing). Presumably, the priest may turn after the consecration in order “to show to the people” the Host and Chalice. He does not turn for the Preface dialogue, the Mystery of Faith, or the Our Father. He has the choice of offering the post-communion prayer and final blessing at the altar rather than the chair (GIRM 122).
It is important to note that the use of celebration ad orientem in the revised liturgy differs from the Tridentine practice of celebration “at the altar” because the opening rites and liturgy of the Word now take place at the chair and lectern. Permission for celebrating the entire Mass facing the people was evidently extended or granted anew because the practice is mentioned as a consideration in the design of altars (GIRM 262).
This brief survey shows that the reforms following Vatican II required the liturgy of the Word to be celebrated versus populum while providing for the continuation of the Church’s immemorial practice of celebrating the liturgy of the Eucharist ad orientem. These basic orientations were established by the liturgical books and were not left to be determined by other ecclesial authorities, such as the Episcopal Conferences.
Permission was explicitly given to priests to celebrate versus populum instead of ad orientem if they wished (much as priests may choose the penitential rite or whether to have a communal exchange of peace). Celebration ad orientem therefore remains in the contemporary liturgy as both an ancient treasure and an example of legitimate liturgical diversity.