parallel bars vs plural

parallel bars and plural both are nouns.

Word	Noun	Adjective	Verb	Adverb
parallel bars	Yes	No	No	No
plural	Yes	Yes	No	No

parallel bars (noun)	plural (noun)
gymnastic apparatus consisting of two parallel wooden rods supported on uprights	the form of a word that is used to denote more than one

parallel bars (adjective)	plural (adjective)
	grammatical number category referring to two or more items or units
	composed of more than one member, set, or kind