keystone vs linchpin

keystone and linchpin both are nouns.

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

As nouns, keystone and linchpin are synonyms defined as:

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

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

keystone (noun)	linchpin (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