2 vs couplet

2 and couplet both are nouns.

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

As nouns, couplet is a hyponym of 2; that is, couplet is a word with a more specific, narrower meaning than 2:

2: the cardinal number that is the sum of one and one or a numeral representing this number
couplet: two items of the same kind

Other hyponyms of 2 include craps, snake eyes, brace, couple, distich, duad, duet, duo, dyad, pair, span, twain, twosome, yoke.

2 (noun)	couplet (noun)
the cardinal number that is the sum of one and one or a numeral representing this number	a stanza consisting of two successive lines of verse; usually rhymed
	two items of the same kind

2 (adjective)	couplet (adjective)
being one more than one