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The Catholic Encyclopedia
That part of the altar which faced the congregation, in contradistinction to the side at which the priest stood when formerly the latter stood at the altar facing the people. In ceremonials we frequently find mention of the right and left side of the altar. Before 1488, the epistle side was called the right side of the altar, and the gospel side the left. In that year, Augustine Patrizi, Bishop of Pienza, published a ceremonial in which the epistle side is called the left of the altar, and the gospel side the right, the denomination being taken from the facing of the cross, the principal ornament of the altar, not of the priest or the laity. This change of expression was accepted by St. Pius V and introduced into the rubrics.
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Obstat, Nihil. Lafort, Remy, Censor. Entry for 'Altar Side'. The Catholic Encyclopedia. https://www.studylight.org/encyclopedias/eng/tce/a/altar-side.html. Robert Appleton Company. New York. 1914.