The quality or state of being decorative; the degree to which something serves to ornament or beautify.
From decorative + -ness (noun-forming suffix creating abstract qualities). The -ness suffix has been productive in English since Old English times. This compound form emerged in the 18th century as aesthetic theory developed.
This noun-ification of 'decorative' shows how much we value being able to talk about qualities as abstract concepts—we don't just say 'it's decorative,' we can now discuss the 'decorativeness' of objects, suggesting decoration became a measurable aesthetic principle rather than just an incidental feature.
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