Blue skull key

The Blue Skullkey is used to open designated locked doors. If the player encounters a locked door and attempts to open it without the correct key, the door will not open and the player will get a message saying so. Once he obtains the key, he can then open any locked blue door on the map. Once you exit a map you lose all of your keys.

Doom also does not differentiate between the Blue Skullkey and the Blue Keycard, though this was later changed first in the Boom source port and is now the norm for most major ports.