perspicuity vs plainness

perspicuity and plainness both are nouns.

Word	Noun	Adjective	Verb	Adverb
perspicuity	Yes	No	No	No
plainness	Yes	No	No	No

As nouns, perspicuity and plainness are synonyms defined as:

perspicuity and plainness: clarity as a consequence of being perspicuous

Other synonyms of perspicuity include perspicuousness.

perspicuity (noun)	plainness (noun)
clarity as a consequence of being perspicuous	the appearance of being plain and unpretentious
	an appearance that is not attractive or beautiful
	clarity as a consequence of being perspicuous
	the state of being unmixed with other material