Several parts combine into one character.

顎 is a phonetic-semantic compound: 頁 (the "head / page" radical, which despite the modern "page" meaning originally depicted a head and is used in characters for facial features) + 咢 (è, "startled cry" — providing the sound). The composite reads as "the lower part of the head" = jaw. The 頁 radical anchors all face-and-head anatomy in CJK: 顔 (face), 頭 (head), 額 (forehead), 頰 (cheek), 顎 (jaw), 頸 (neck). Mainland simplified to 颚.

Korean reading "ak." 上顎 (sangak, upper jaw / maxilla — anatomical), 下顎 (haak, lower jaw / mandible), 顎關節 (akgwanjeol, temporomandibular joint — TMJ, the jaw joint that often causes problems and is discussed in Korean dental health contexts). Korean uses 악 strictly in medical and dental terminology; everyday "jaw" is the native (teok).

Mandarin è, 4th tone (simplified 颚). 上颚 (shàng'è, upper jaw), 下颚 (xià'è, lower jaw), 颚骨 (ègǔ, jawbone). Modern Mandarin uses these in medical and zoological contexts; everyday speech for "chin / jaw" is 下巴 (xiàba).

Japanese on-reading ガク (gaku) — 上顎 (jōgaku, upper jaw), 下顎 (kagaku, lower jaw — formal medical term). Kun-reading あご (ago) is the everyday Japanese word for jaw / chin. The Japanese idiom 顎が落ちる (ago ga ochiru, "the jaw falls off") means "to be astonishingly delicious" — a vivid expression for food so good your jaw drops in awe. Used in food reviews: あごが落ちるほど美味しい ("delicious enough to drop your jaw"). Another: 顎で使う (ago de tsukau, "to use someone with the jaw") = "to order someone around imperiously" — the gesture of pointing with the chin to dismiss or command.

Memory aid: head-radical (頁) plus 咢 (sound) — the part of the head where the lower part hinges = jaw.