Big Key

Big Keys are large key items that appear in The Legend of Zelda: A Link to the Past, The Legend of Zelda: The Wind Waker, The Legend of Zelda: Four Swords Adventures, The Legend of Zelda: The Minish Cap, and The Legend of Zelda: A Link Between Worlds. Big Keys are similar to Boss Keys as they are required to open a boss door, but Big Keys can also open large Treasure Chests in some appearances. However, aside from Four Swords Adventures, later games change them to be more like Boss Keys, suggesting they have become or always have been the same item. In fact, The Legend of Zelda: Spirit Tracks has a Boss Key be referred to generically as a "big key" by Princess Zelda multiple times.

The Legend of Zelda: A Link to the Past
In The Legend of Zelda: A Link to the Past, Big Keys have details resembling horns and a pair of angry eyes. One appears in each dungeon, and can be used repeatedly if need be. The first one is found in Hyrule Castle, being carried by the Ball & Chain Trooper, and is used to unlock Zelda's cell. Later, they are found in other dungeons, where they open doors with giant locks that lead deeper into the dungeon, though unlike later games it typically does not directly lead to the boss's room. Additionally, they open the giant treasure chests containing the dungeon's primary item.

The Legend of Zelda: The Wind Waker
In The Legend of Zelda: The Wind Waker, Big Keys are now functionally identical to Boss Keys in the Nintendo 64 games in that they are found in a large, ornate chest and open a doorway straight to the boss. They use a purple, horned eye design that is used for Boss Keys in The Legend of Zelda: Phantom Hourglass and The Legend of Zelda: Spirit Tracks.

The Legend of Zelda: Four Swords Adventures
In The Legend of Zelda: Four Swords Adventures, Big Keys are found in a few stages and look how they do in A Link to the Past. In this game, they must be physically carried to be used, and are only used to open giant chests, which contain huge Force Gems.

The Legend of Zelda: The Minish Cap
In The Legend of Zelda: The Minish Cap, Big Keys have their appearance and role from The Wind Waker.

The Legend of Zelda: A Link Between Worlds
In The Legend of Zelda: A Link Between Worlds, Big Keys look like they do in A Link to the Past, but are found in large treasure chests and only used to open large-locked doors.