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The Wario Cap (or Wario's cap) is Wario's traditional headwear. The hat is yellow, with a large blue "W" emblem inside a small white circle and a white rim. It has been featured in every Wario Land game, and many other appearances of Wario elsewhere (a notable exception is its absences in the WarioWare series, as Wario wore a biking outfit in that series, instead of his classic clothing).
The Wario Cap was based fully on the Mario Cap with minor differences. The cap got its yellow color artwork from Super Mario Land 2: 6 Golden Coins, but it never got colored that way in games until Wario's Woods, for every game he appeared in up until that point were black and white Game Boy games. Like his rival, Wario has been seen wearing the cap since he was a baby as seen in Yoshi's Island DS.
If Wario loses in Mario Kart 64 and Mario Kart: Super Circuit, his profile picture will show him biting his cap. This is a pun on the phrase "I'll eat my hat"- used to express strong disbelief for something, in this case Wario losing.
In Super Mario 64 DS, Wario's Cap affects Wario as Mario's Cap affects Mario. If Wario loses his cap at any time, his defense and strength will sharply decline, which can only be restored by getting it back. While his cap is absent, Wario is unable to pick up any caps or turn into Metal Wario. Also in that same game, Wario's Cap could be picked up by either Mario, Luigi, or Yoshi to shape-shift into Wario. Until he is unlocked, the only course his cap appears in is Hazy Maze Cave. If he loses his cap and gains a star from another level, Wario can simply talk to Toad (who will find and be wearing Wario's Cap). While the cap is important to Wario, Toad diminishes it by saying that it "reeks pretty bad" and that Wario should wash it out more often.
In Wario: Master of Disguise the hat plays a more important part as a checkpoint in each chapter. There are many hats scattered in each chapter and if the player finds one, they can save the game. By saving, the player won't have to start from the beginning of the level if they die, instead starting from the location of the checkpoint hat.
The Wario Cap can be obtained for Mario to wear in Super Mario Odyssey. It can either be purchased from a Crazy Cap store or obtained through scanning a Wario amiibo (the latter also unlocks Wario's costume).
The Wario Cap hasn't always been its classic yellow as seen in most of the games Wario appears in. In some segments of Wario's Woods his cap appears purple, although in the SNES version, his cap appears yellow. In Wario Land II and Wario Land 3, the Wario Cap is depicted as white in-game. In Super Mario Land 2: 6 Golden Coins and Wario Land: Super Mario Land 3 his cap was gray due to the monochrome display of the Game Boy. In Wario Land 4, his cap is depicted as yellow like his official artwork.
The cap has also been imitated on different headgear Wario has worn. Wario's WareWare helmet is in the style of his cap with the white rim and blue "W" emblem. Wario's safari hat from Wario Land also has his signature blue "W" emblem.