See caption
Standard return quoin, with one long side over one short side
Source: Designing Buildings Wiki

A quoin is any external angle or corner of a structure,[1] often decorated with quoins – brick or stonework that projects from the face of the wall.[2] The term derives from the French coin, meaning “corner”.[3]

Historically, the term quoin was also used by printers for a small expanding device that held the typeset firmly in place in a frame known as a forme.[4]