Boss Key
The Boss Key, a.k.a the Big Key, is an item found in many Legend of Zelda games. It usually allows the player to unlock the door to the boss chamber. This key commonly appears bigger and more ornate than other keys, and is sometimes customized to fit the dungeon theme. In each Zelda game, the Boss Key, or Big Key, has a slightly different use or appearance. All of the Big Key's appearance are listed below.
A Link to the Past
In this game there was a Big Key. This key was used to open up a Big Chest, which contained the dungeon item. It also opened a door that lead to the boss.
Link's Awakening
The Legend of Zelda: Link's Awakening had a different version of the Big Key that was called the Nightmare Key. It had the only function of opening up the door that lead directly to the boss chamber.
Ocarina of Time
Ocarina of Time was the first game to introduce 3D perseptive, and thus we got to see the Boss Key in 3D for the first time. The Boss Key opened the door that lead directly to the boss chamber.
Majora's Mask
The Boss Key in Majora's Mask served the same purpose as the Boss Key from Ocarina of Time (see above)
Oracle of Ages
The Boss Key that was present in Oracle of Ages had the exact same use as in Link's Awakening (see above).
Oracle of Seasons
Considering Oracle of Ages and Oracle of Seasons were released at the same time, the developers recycled some ideas. The Boss Key in this game had the exact same function as in Oracle of Ages (see above)
The Wind Waker
The Minish Cap
In each and every dungeon in The Minish Cap, there featured a Boss Key that's only use was to open the door to the boss chamber.
Twilight Princess
The Boss Key featured in Twilight Princess had the exact same use as the one from Ocarina of Time (see above).
Phantom Hourglass
Spirit Tracks
The Boss Key from Spirit Tracks had the one and only use of opening the door to the boss chamber. However, the Boss Key in this game had a little twist. Link could not store the Key in his tunic, but instead had to carry it from where he found it to the lock. This means that he couldn't use his sword while walking.
Skyward Sword
In Skyward Sword, the traditional Boss 'Key' has been removed, and in its place is a sculpture. With this sculpture, Link must insert it into the lock, but it only goes in one way, so it can get confusing.