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The Yellow Key is a key found in Wario: Master of Disguise. It is one of three colored keys found in Allergia Gardens, the other two being the Red Key and Blue Key. The purple treasure chest containing it is in the isolated center of a room with Ratfaels and is accessed by turning the blue door in the bottom-right corner of said room green, then turning the blue door in the following room green. The Yellow Key opens two locked doors of the same color.
After Wario obtains the key, the number 2 appears next to the key's icon on the top screen. After unlocking one of the doors, the number changes to a 1, implying that the Yellow Key is actually two keys. However, when obtained, Wario refers to the key as a singular entity. It shares this trait with the Red Key and Master Key.