Key (Luigi's Mansion series)
Keys are a recurring item within the Luigi's Mansion series. They appear in every installment except Luigi's Mansion Arcade. Keys share a similar purpose to the items of the same name within The Legend of Zelda series in that they have a one-time use of unlocking a specific locked door in a mansion, substituting for dungeons in The Legend of Zelda games.
In Luigi's Mansion, Luigi has to collect many keys before he could advance to other rooms. They were usually found in Treasure Chests, although some could be found right out in the open. Also, there are four special keys, with each one having a different card suit: heart, club, diamond, and spade.
Luigi's Mansion: Dark Moon
Keys return in Luigi's Mansion: Dark Moon, with a different design. They're again used to open doors and any key can be used to unlock a door. In the Old Clockworks, a key can be generated from a gold pot.
When a Key is used to unlock a door, it inexplicably turns into dust. The only exception for this rule is the Front-Door Key of the Treacherous Mansion.