mainstay vs keystone

mainstay and keystone both are nouns.

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

As nouns, mainstay and keystone are synonyms defined as:

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

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

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