keystone vs mainstay

keystone and mainstay both are nouns.

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

As nouns, keystone and mainstay are synonyms defined as:

keystone and mainstay: a central cohesive source of support and stability

Other synonyms of keystone include anchor, backbone, linchpin, lynchpin.

keystone (noun)	mainstay (noun)
the central building block at the top of an arch or vault	the forestay that braces the mainmast
a central cohesive source of support and stability	a central cohesive source of support and stability
	a prominent supporter