A coign (also written quoin, coigne, and coin) is the exterior angle of a wall, tower, keep, etc. and all component stones of the angle. Coign stones tend to be larger than the stones of the wall proper, and lend an aesthetic to the structure. A coign is also the term for a keystone in an arched doorway. The origin of the word is from the French coin, meaning "corner".

In printwork, a coign is a wooden or metal wedge to lock the type in a chase.

In the context of missle weapons, a coign is a wedge used to raise the angle of a gun or other projectile weapon.