Where the shape comes from
A mouth drawn wide open with a neat row of teeth inside it — one of the more literal pictographs among the radicals — until 止 zhǐ was fitted on top purely to fix the pronunciation, close to 齒’s own old sound, leaving the teeth themselves compressed into the lower half of the character. Because tooth eruption and wear were the ordinary way to judge an animal’s or a person’s age, 齒 extended early to age itself and further to rank, to be counted alongside, as in 不足挂齿, not worth mentioning, literally not worth hanging on the teeth. It names the everyday word in 牙齿, teeth, working alongside 牙 (radical 92, the individual fang or tusk) for the fuller set, the gum-line, or the more technical vocabulary, and gives 齿轮, a gear, its cogs pictured as a wheel’s row of teeth.
汉字 filed under 齒
No published character on HSK Guide is filed under this radical yet. 汉字库.