perspicuousness vs plainness

perspicuousness and plainness both are nouns.

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

As nouns, perspicuousness and plainness are synonyms defined as:

perspicuousness and plainness: clarity as a consequence of being perspicuous

Other synonyms of perspicuousness include perspicuity.

perspicuousness (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