plainness vs lucidity

plainness and lucidity both are nouns.

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

As nouns, lucidity is a hypernym of plainness; that is, lucidity is a word with a broader meaning than plainness:

plainness: clarity as a consequence of being perspicuous
lucidity: free from obscurity and easy to understand; the comprehensibility of clear expression

Other hypernyms of plainness include clarity, clearness, limpidity, lucidness, pellucidity.

plainness (noun)	lucidity (noun)
the appearance of being plain and unpretentious	free from obscurity and easy to understand; the comprehensibility of clear expression
an appearance that is not attractive or beautiful	a lucid state of mind; not confused
clarity as a consequence of being perspicuous
the state of being unmixed with other material