Napier's bones vs plural

Napier's bones and plural both are nouns.

Word	Noun	Adjective	Verb	Adverb
Napier's bones	Yes	No	No	No
plural	Yes	Yes	No	No

Napier's bones (noun)	plural (noun)
a set of graduated rods formerly used to do multiplication and division by a method invented by John Napier	the form of a word that is used to denote more than one

Napier's bones (adjective)	plural (adjective)
	grammatical number category referring to two or more items or units
	composed of more than one member, set, or kind