What is "perpend 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.

Perpend stone (parpen, parpend, perpin, and other spellings), bond stone, or tie stone is a structural element building term used by stonemasons and brick masons. Perpend is also a piece in brickwork also called a cross joint or when extending through the entire wall a transverse joint or perpend bond.

Usually stone walls are built with two layers of stone, an inner and an outer layer, with the space between them sometimes filled with rubble. A perpend stone is a longer stone that extended through the entire wall's width, from the outer wall to inner wall, which serves to lock the two wall layers structurally together.