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The Catholic Encyclopedia
On the Jewish altar there were four projections, one at each corner, which were called the horns of the altar. These projections are not found on the Christian altar, but the word cornu ("horn") is still maintained to designate the sides or corners of the altar. Hence cornu epistolae and cornu evangelii mean the epistle and gospel side of the altar respectively, cornu anterius and cornu posterius evangelii or cornu dexterum anterius and dexterum posterius mean respectively the anterior or posterior corner of the altar at the gospel side.
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Obstat, Nihil. Lafort, Remy, Censor. Entry for 'Altar Horns'. The Catholic Encyclopedia. https://www.studylight.org/encyclopedias/eng/tce/a/altar-horns.html. Robert Appleton Company. New York. 1914.