What is "perpender"

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.

n. A large stone reaching through a wall so as to appear on both sides of it, and acting as a binder.