Several parts combine into one character.

鍵 is a phonetic-semantic compound: 金 (metal) + 建 (jiàn, "to set up / establish"). The original meaning was "the metal bar that locks a door" — the rod that, when set in place, secured a lock. From this concrete image three meanings unfurled: (1) key — the physical implement; (2) the crux / essential point — by metaphorical extension, that which "unlocks" understanding; (3) keyboard / piano key — the metal levers pressed to produce sound or input. The 金 radical anchors the entire CJK metalworking vocabulary: 鏡 (mirror), 鉄 (iron), 針 (needle), 銀 (silver), 銅 (copper), 鋼 (steel).

The simplified Mainland form is 键, with the same component structure but a more abbreviated metal radical.

Korean reading "geon." Largely formal in modern Korean — most everyday "key" usage prefers the native (yeolsoe) or English-derived (ki). The Sino-Korean compound that survives prominently is 鍵盤 (geonban, "key-board" = piano keyboard or computer keyboard). Korean musicians use 건반 to refer to piano keys; computer users say 키보드 instead.

Mandarin jiàn, 4th tone (simplified 键). The character's "essential / crucial" meaning dominates modern Chinese: 关键 (guānjiàn, "passage-key" = critical point / crux — extremely common), 关键时刻 (guānjiàn shíkè, critical moment), 关键词 (guānjiàncí, keyword). 键盘 (jiànpán, keyboard) and 按键 (ànjiàn, button / key on a device) are everyday tech terms. The everyday Chinese word for a physical key is 钥匙 (yàoshi), not 键 — modern Chinese reserves 键 for keyboards and the metaphorical "crux."

Japanese on-reading ケン (ken) — 鍵盤 (kenban, keyboard). Kun-reading かぎ (kagi) is the everyday word: 鍵 (kagi, key), 鍵穴 (kagiana, "key-hole" = keyhole), 合鍵 (aikagi, "matching key" = duplicate key / spare key). Modern Japanese also uses 鍵 metaphorically: 成功の鍵 (seikō no kagi, "the key to success") parallels English idiom precisely. Smartphone unlock metaphors live here: スマホの鍵を解除 (sumaho no kagi o kaijo, "to release the smartphone's lock").

Memory aid: metal (金) erected / set up (建) — a metal rod set in place to secure a lock. From physical key, the metaphor extends to crux and keyboard.