Several parts combine into one character.

牙 is a pictograph of "two teeth biting each other at an angle" — an oracle-bone character resembling a sideways "ヲ" shape, depicting the interlock of upper and lower molars. 牙 pairs with 齒 (chǐ, "incisors / a row of front teeth"): 齒 = the row of front teeth visible when smiling, 牙 = the back molars or animal fangs. A division of labor inside CJK's tooth vocabulary.

Korean reading "a." 象牙 (sang-a, ivory — literally "elephant-tooth"), 齒牙 (chi-a, teeth — formal medical Korean), 牙城 (a-seong, "fortress of fangs" = a stronghold / power base). 牙城 is a vivid Korean expression that turns a wild animal's lair into a political concept — Korean newspaper headlines use ("the conservative stronghold") as a stock phrase for entrenched political bases.

Mandarin yá, 2nd tone. 牙 (yá, tooth — daily), 牙齿 (yáchǐ, teeth), 牙刷 (yáshuā, toothbrush), 牙膏 (yágāo, toothpaste), 象牙 (xiàngyá, ivory). Mandarin uses 牙 as the everyday word for "tooth" — and this is one of the most striking divergences in shared CJK script: Korean uses Hangul 이, Japanese uses 歯 (ha), Mandarin uses 牙. Three different daily words for the same body part across three cultures that share the writing system.

Japanese on-readings ガ (ga) and ゲ (ge) — 象牙 (zōge, ivory), 歯牙 (shiga, tooth — formal). Kun-reading きば (kiba) — but here Japanese makes a sharp distinction: 牙 (kiba) means specifically "fang / animal tooth," not human tooth. 狼の牙 (ōkami no kiba, "the wolf's fang") is a stock phrase in Japanese folklore. Japanese reserves 牙 (kiba) for animal teeth and uses 歯 (ha) for human teeth — a distinction the daily Mandarin word 牙齿 collapses entirely.

Memory aid: three CJK languages, three different everyday words for "tooth" — Korean 이, Japanese 歯, Mandarin 牙 — a vivid case where shared writing diverges into completely different daily speech.