plainness vs restraint

plainness and restraint both are nouns.

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

As nouns, restraint is a hyponym of plainness; that is, restraint is a word with a more specific, narrower meaning than plainness:

Other hyponyms of plainness include chasteness, simpleness, simplicity, austereness, severeness, severity, bareness, starkness.

plainness (noun)	restraint (noun)
the appearance of being plain and unpretentious	the act of controlling by restraining someone or something
an appearance that is not attractive or beautiful	a device that retards something's motion
clarity as a consequence of being perspicuous	discipline in personal and social activities
the state of being unmixed with other material	lack of ornamentation
	a rule or condition that limits freedom
	the state of being physically constrained