letter key

例句：

1. 1. The old man used his letter keyto unlock the door.
   老人用他的钥匙打开了门。
2. 2. She kept her letter key inher purse for easy access.
   她把钥匙放在钱包里以便随时取用。
3. 3. Theletter key was a crucial part of thesecurity system.
   这把钥匙是安全系统的关键部分。