signatory vs cosigner

signatory and cosigner both are nouns.

Word	Noun	Adjective	Verb	Adverb
signatory	Yes	No	No	No
cosigner	Yes	No	No	No

As nouns, cosigner is a hyponym of signatory; that is, cosigner is a word with a more specific, narrower meaning than signatory:

signatory: someone who signs and is bound by a document
cosigner: one of two or more signers of the same document (as a treaty or declaration)

Other hyponyms of signatory include cosignatory, abhorrer, endorser, indorser, attestant, attestator, attestor, witness.

signatory (noun)	cosigner (noun)
someone who signs and is bound by a document	one of two or more signers of the same document (as a treaty or declaration)
	a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)