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key
lock
key (to a problem, success, etc.); clue
「鍵」 has another reading with different meanings.
key (on a piano, organ, etc.)
鍵かぎをどこに置いおいたか忘れわすれた。
I forgot where I put the key.
出かけるでかけるときは必ずかならず鍵かぎをかけてね。
Make sure to lock the door when you go out.
スペアの鍵かぎを作っつくっておいたほうがいいよ。
You should have a spare key made.