The word "扣子" (kòuzi), meaning "button", is a standard Mandarin compound noun formed through a common morphological process. It consists of the meaningful root "扣" (kòu), which as a verb means "to button", "to fasten", or "to deduct", and here specifically conveys the action of fastening. This is combined with the nominalizing suffix "子" (zi), a frequently used, often toneless suffix that transforms verbs or other roots into concrete, everyday nouns. Therefore, the formation follows the pattern of Verb/Meaningful Root + Nominal Suffix → Concrete Noun, creating a word that literally means "the thing used for fastening". This structure is analogous to similar words like "刷子" (shuāzi, brush, from "to brush") and "夹子" (jiāzi, clip, from "to pinch").