Mask

Masks have been a common feature in The Legend of Zelda series since Ocarina of Time. When worn, the masks give Link a variety of abilities. Some of these abilities are required to complete the game, others are used for optional sidequests, and others are available just as fun items to collect.

Majora's Mask uses the masks as one of the primary gameplay features. Four masks allow Link to change his form entirely. While in another form, Link has access to abilities that are unavailable to him otherwise.

Ocarina of Time
The concept of the Mask Trading Sequence is that, when Link visits the Happy Mask Shop, the Happy Mask Salesman will lend him a mask. Link then looks for a character in Hyrule who wants the mask, and sells it for an amount of money. Link then goes back to the shop and pays for the price of the mask, and is given the following mask. All of these masks are obtainable and usable only during the child era. The masks listed are ordered by appearance.

Majora's Mask
The masks are ordered this way: Transformation Masks, Happy Masks (listed alphabetically), and Boss masks.

Other games
After Majora's Mask, masks have become far less prominent.

In Freshly-Picked Tingle's Rosy Rupeeland, Tingle receives a mask for defeating Baron, not unlike the Boss Remains in Majora's Mask. The origin of this mask is debated, with a different story told by each Bodyguard, and is immediately sold to that Bodyguard. It has no purpose beyond this financial reward.