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Cyclopedia of Biblical, Theological and Ecclesiastical Literature
is a common heraldic bearing in the insignia of sees and religious houses, particularly such as are under the patronage of St. Peter. Two keys in salire are frequent, and keys are sometimes interlaced or linked together at the bows, i.e. rings. Keys indorsed are placed side by side, the wards away from each other.
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McClintock, John. Strong, James. Entry for 'Key'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. https://www.studylight.org/encyclopedias/eng/tce/k/key.html. Harper & Brothers. New York. 1870.