How did 優しさ derived from 優しい? The rule is？


How did 優しさ derived from 優しい? The rule is？




> 優し **さ**

The suffix (接尾辞)「さ」, attached to the stem (語幹) of i-adjectives and na-
adjectives, turns them into the noun form, like this:

> i-adjective おおきい "big" - stem おおき + suffix さ → おおきさ "size"  
>  na-adjective にぎやかな "lively" - stem にぎやか + suffix さ → にぎやかさ "liveliness"

A few more examples:

> i-adjectives:  
>  かわいい "cute" → かわいさ "cuteness"  
>  [長]{なが}い "long" → 長さ "length"  
>  [明]{あか}るい "bright" → 明るさ "brightness"
>
> na-adjectives:  
>  きれいな "clean" → きれいさ "cleanness"  
>  [華]{はな}やかな "brilliant" → 華やかさ "brilliance"  
>  [便利]{べんり}な "convenient" → 便利さ "convenience"

