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The Collaborative International Dictionary
perpent stone

Perpender \Per*pend"er\, n. [F. parpaing, pierre parpaigne; of uncertain origin.] (Masonry) A large stone reaching through a wall so as to appear on both sides of it, and acting as a binder; -- called also perbend, perpend stone, and perpent stone.

Wiktionary
perpent stone

n. A perpender.