closed couplet vs couplet

closed couplet and couplet both are nouns.

Word	Noun	Adjective	Verb	Adverb
closed couplet	Yes	No	No	No
couplet	Yes	No	No	No

As nouns, couplet is a hypernym of closed couplet; that is, couplet is a word with a broader meaning than closed couplet:

closed couplet: a rhymed couplet that forms a complete syntactic unit
couplet: a stanza consisting of two successive lines of verse; usually rhymed

closed couplet (noun)	couplet (noun)
a rhymed couplet that forms a complete syntactic unit	a stanza consisting of two successive lines of verse; usually rhymed
	two items of the same kind