The condition of being plain (in all senses)(noun)
The product or result of being plain(noun)
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Use "plainness" in a sentence
"There are two differing roots for this term: Some claim it refers to the plainness of vanilla ice cream as opposed to those who like more adventurous flavors; others believe it comes from techno-culture, where a “vanilla” PC means a very basic machine, without bells and whistles."
"I think Presbyterians have erred in plainness in ecclesiastical life."
"Let pride, which she calls plainness, marry her! ["