Versus populum
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Versus populum is the liturgical stance of a priest who, while celebrating Mass, faces the people from the other side of the altar. The opposite stance, that of a priest facing in the same direction as the people, is today called ad orientem or ad apsidem.
A Roman Rite Novus Ordo Pontifical Mass celebrated versus populum in Marikina, Philippines
A priest facing the congregation as he says Ecce Agnus Dei at a Solemn Tridentine Mass.
Ad orientem
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Ad orientem, meaning "to the east" in Ecclesiastical Latin, is a phrase used to describe the eastward orientation of Christian prayer and Christian worship, comprising the preposition ad (toward) and oriens, participle of orior.
Tridentine Mass, celebrated regularly ad orientem
A Christian cross hanging on the eastern wall of a modern house, indicating the eastward direction towards which prayer is focused
Eastern Orthodox Christian pilgrims making prostrations at Golgotha in the Church of the Holy Sepulchre, Jerusalem