-ness is a suffix you add to the end of adjectives to create nouns that name a quality or state of being. When you add -ness to “dark,” you get “darkness,” which names the quality of being dark. Add it to “kind” and you get “kindness,” the quality of being kind.

This tiny ending transforms describing words into naming words. “Sad” describes how someone feels, but “sadness” names the feeling itself. “Rough” describes a surface, but “roughness” names that bumpy quality you can feel with your fingers.

You'll find -ness everywhere in English. Think of happiness, wilderness, brightness, loneliness, or forgiveness. Each one takes an adjective (happy, wild, bright, lonely, forgiving) and turns it into a thing you can discuss, measure, or experience. You might talk about the thickness of a book, the sweetness of honey, or the strangeness of a dream.

Sometimes adding -ness requires small spelling changes. “Happy” becomes “happiness” (the y changes to i), and “tidy” becomes “tidiness” (the y changes to i).