couplet vs pair

couplet and pair both are nouns.

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

As nouns, couplet and pair are synonyms defined as:

Other synonyms of couplet include brace, couple, distich, duad, duet, duo, dyad, span, twain, twosome, yoke.

couplet (noun)	pair (noun)
a stanza consisting of two successive lines of verse; usually rhymed	two people considered as a unit
two items of the same kind	a set of two similar things considered as a unit
	a poker hand with 2 cards of the same value
	two items of the same kind

couplet (verb)	pair (verb)
	engage in sexual intercourse
	bring two objects, ideas, or people together
	arrange in pairs
	occur in pairs
	form a pair or pairs