Quoin /(kwoin [or] koin; 277)/

Quoin

n.
  1. Originally, a solid exterior angle, as of a building; now, commonly, one of the selected pieces of material by which the corner is marked. (Arch.)
  2. A wedgelike piece of stone, wood, metal, or other material, used for various purposes; (Masonry)

Phrases & Compounds
Hollow quoin
See under Hollow.
Quoin post
the post of a lock gate which abuts against the wall.