keystone vs quoin

keystone and quoin both are nouns.

Word	Noun	Adjective	Verb	Adverb
keystone	Yes	No	No	No
quoin	Yes	No	No	No

As nouns, quoin is a hyponym of keystone; that is, quoin is a word with a more specific, narrower meaning than keystone:

Other hyponyms of keystone include coign, coigne.

keystone (noun)	quoin (noun)
the central building block at the top of an arch or vault	(architecture) solid exterior angle of a building; especially one formed by a cornerstone
a central cohesive source of support and stability	the keystone of an arch
	expandable metal or wooden wedge used by printers to lock up a form within a chase