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1911 Encyclopedia Britannica
RETRO-CHOIR, a space behind the high altar in the choir of a church, in which there is, or was, a small altar standing back to back with the other.
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Chisholm, Hugh, General Editor. Entry for 'Back-Choir'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/b/back-choir.html. 1910.