Which item fits together with "key"?

Explanation

The use of locks with keys can be traced many hundred years back in time. In ancient China, very early locking mechanisms has been discovered, and in medieval Europe, locks where used to lock the town gates among other things. Today, there are locking mechanisms in many places such as buildings, cars and cupboards. A typical distinction is made between a padlock and door lock: The padlock can be moved as needed, while the door lock is built in or placed next to the door.