Versus populum  Versus populum (Latin for "towards the people") is the liturgical orientation in which the priest celebrates Mass facing the people. The opposite orientation, whereby the priest faces in the same direction as the people (thereby having his back to them), is called "ad orientem" ("towards the east" − even if the priest is not facing eastward) or "ad apsidem" ("towards the apse" − even if the altar is unrelated to the apse of the church or even if the church or chapel has no apse). In the early history of Christianity it was considered the norm to pray facing the geographical east. From the middle of the 17th century, almost all new Roman Rite altars were built against a wall or backed by a reredos, with a tabernacle placed on the main altar or inserted into the reredos. This meant that the priest turned to the people, putting his back to the altar, only for a few short moments at Mass. However, the Tridentine Missal itself speaks of the option of celebrating "versus populum", and gives corresponding instructions for the priest when performing actions that in the other orientation involved turning around in order to face the people. It has been said that the reason the Pope always faced the people when celebrating Mass in St Peter's was that early Christians faced eastward when praying and, due to the difficult terrain, the basilica was built with its apse to the west. Some have attributed this orientation in other early Roman churches to the influence of Saint Peter's. However, the arrangement whereby the apse with the altar is at the west end of the church and the entrance on the east is found also in Roman churches contemporary with Saint Peter's (such as the original Basilica of Saint Paul Outside the Walls) that were under no such constraints of terrain, and the same arrangement remained the usual one until the sixth century. According to Klaus Gamber, in this early layout the people were situated not in the central nave but in the side aisles of the church and, while the priest faced both the altar and east throughout the Mass, the people faced the altar (from the sides) until the high point of the Mass, when they would turn to face east, the direction in which the priest was already facing. This view is strongly criticized on the grounds of the unlikelihood that, in churches where the altar was to the west, they would turn their backs on the altar (and the priest) at the celebration of the Eucharist.