linchpin vs keystone

linchpin and keystone both are nouns.

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

As nouns, linchpin and keystone are synonyms defined as:

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

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

linchpin (noun)	keystone (noun)
pin inserted through an axletree to hold a wheel on	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