琴 ('stringed/keyboard instrument') +
键 ('key, like a key on a lock') = 'instrument key.' The same
键 turns up in
键盘 (jiànpán, 'keyboard') and
按键 (ànjiàn, 'press a button'). Don't confuse
琴键 (instrument key) with
键盘 (computer/instrument keyboard as a whole) or
钥匙 (yàoshi, 'door key').
