join vs mortise

join and mortise both are nouns.

Word	Noun	Adjective	Verb	Adverb
join	Yes	No	Yes	No
mortise	Yes	No	Yes	No

As verbs, mortise is a hyponym of join; that is, mortise is a word with a more specific, narrower meaning than join:

join (noun)	mortise (noun)
a set containing all and only the members of two or more given sets	a square hole made to receive a tenon and so to form a joint
the shape or manner in which things come together and a connection is made

join (verb)	mortise (verb)
make contact or come together	join by a tenon and mortise
cause to become joined or linked	cut a hole for a tenon in
become part of; become a member of a group or organization
come into the company of
be or become joined or united or linked