keystone vs lynchpin

keystone and lynchpin both are nouns.

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

As nouns, keystone and lynchpin are synonyms defined as:

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

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

keystone (noun)	lynchpin (noun)
the central building block at the top of an arch or vault	pin inserted through an axletree to hold a wheel on
a central cohesive source of support and stability	a central cohesive source of support and stability