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]

References

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Bibliography

Clarke, Michael. “Quoin.” Concise Oxford Dictionary of Art Terms, Online, Oxford University Press, 2010, https://www.oxfordreference.com/view/10.1093/acref/9780199569922.001.0001/acref-9780199569922-e-1413.
Curl, James Stevens, and Susan Wilson. “Quoin.” A Dictionary of Architecture and Landscape Architecture, 3rd ed., Oxford University Press, 2015, https://www.oxfordreference.com/view/10.1093/acref/9780199674985.001.0001/acref-9780199674985-e-376.
Gorse, Christopher, et al. “Quoins.” A Dictionary of Construction, Surveying and Civil Engineering, Oxford University Press, 2020, https://www.oxfordreference.com/view/10.1093/acref/9780198832485.001.0001/acref-9780198832485-e-5528.
OED. “Quoin, n.” Oxford English Dictionary, Online, Oxford  University Press, 2021, https://www.oed.com/view/Entry/156875.