headstone vs coign

headstone and coign both are nouns.

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

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

Other hyponyms of headstone include coigne, quoin.

headstone (noun)	coign (noun)
the central building block at the top of an arch or vault	the keystone of an arch
a stone that is used to mark a grave	expandable metal or wooden wedge used by printers to lock up a form within a chase