keystone vs headstone

keystone and headstone both are nouns.

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

As nouns, keystone and headstone are synonyms defined as:

keystone and headstone: the central building block at the top of an arch or vault

Other synonyms of keystone include key.

keystone (noun)	headstone (noun)
the central building block at the top of an arch or vault	the central building block at the top of an arch or vault
a central cohesive source of support and stability	a stone that is used to mark a grave