合鍵

1. Origins, structure, and basic meaning

• Structure: 合鍵 is read as あいかぎ and means a duplicate or spare key that opens the same lock as the original.
• Basic usage: It can be a room key left with family or a key labeled to show which door it opens.

2. Common pitfalls and similar expressions

• Leaving a spare key with someone
  • 部屋の合鍵 is a spare key able to open the room. Example: 部屋の合鍵を封筒に入れて家族に預けました。
• Identifying the matching door
  • どの扉の合鍵か asks which door the duplicate key belongs to. Example: 合鍵がどの扉のものか札に書いてあります。