In its guidelines for the arrangement of churches, the current Roman Missal directs: "The altar should be built apart from the wall, in such a way that it is possible to walk around it easily and that Mass can be celebrated at it facing the people, which is desirable wherever possible." Accordingly, altars at which the priest had to face away from the congregation have generally, if this was possible and appropriate, been moved away from the wall or reredos or a new freestanding altar is that on which Mass is celebrated. As a result, in the Roman-Rite Mass the priest usually faces the people. This is not obligatory: the "ad orientem" position is used either by choice, especially for the Tridentine form, or by necessity, due to the position of the altar as in small chapels or oratories.