couplet vs span

couplet and span both are nouns.

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

As nouns, couplet and span are synonyms defined as:

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

couplet (noun)	span (noun)
a stanza consisting of two successive lines of verse; usually rhymed	the act of sitting or standing astride
two items of the same kind	a structure that allows people or vehicles to cross an obstacle such as a river or canal or railway etc.
	the distance or interval between two points
	a unit of length based on the width of the expanded human hand (usually taken as 9 inches)
	two items of the same kind
	the complete duration of something

couplet (verb)	span (verb)
	to cover or extend over an area or time period