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In architecture, both the external corner of a building and, more often, one of the stones used to form that corner. These stones are both structural and decorative in that they often differ in jointing, colour, texture, or size from the masonry of the adjoining walls. Usually quoins are toothed (i.e., set in short courses in a regular pattern of alternating lengths). Such construction dates back to ancient Rome.
This entry comes from Encyclopædia Britannica Concise. For the full entry on quoin, visit Britannica.com.