what is a quoin
Definition of QUOIN. 1. a: a solid exterior angle (as of a building) b: one of the members (as a block) forming a quoin and usually differentiated from the adjoining walls by material, texture, color, size, or projection.2. : the keystone or a voussoir of an arch.: a solid exterior angle (as of a building) b: one of the members (as a block) forming a quoin and usually differentiated from the adjoining walls by material, texture, color, size, or projection.