headstone vs quoin

headstone and quoin both are nouns.

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

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

Other hyponyms of headstone include coign, coigne.

headstone (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 stone that is used to mark a grave	the keystone of an arch
	expandable metal or wooden wedge used by printers to lock up a form within a chase