Palace Key

A Palace Key is a level-exclusive item needed for progress in the Paper Mario series. Palace Keys can come in several varieties.

Paper Mario
In Paper Mario, a Palace Key appears in Crystal Palace along with a Red Key and a Blue Key.

After Mario and co make great progress in Crystal Palace, they encounter a room with two locked doors on the east side. Mario uses the hole on the "mirror" in this room to go to the area behind the mirror, and eventually encounters a small Albino Dino statue in a hallway. After Mario pushes it, the big Albino Dino statue on the south side moves along. Mario then goes back to the south side to access the area below the big Albino Dino statue, and finds a Palace Key in a chest at the end of the path. After Mario uses the key on one of the locked doors, they get both unlocked, allowing Mario to access the Albino Dino riddle room to the next.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, there are two kinds of Palace Keys. The white Palace Keys open doors in the Palace of Shadow, while the blue Palace Keys open pedestals in the Riddle Tower.

There are three Palace Keys of the first kind. The first one is in the area before the Riddle Tower. It is dropped by the Dark Bones in the big Dry Bone room. After Mario catches the Dark Bones and defeats it, it drops the key, therefore allowing Mario to make progress. The second is in the east Palace of Shadow area that leads to Shadow Queen's room (which can be accessed after using the eight Palace Keys of the second kind). In the room to the north of the descended hallways 2 and 3, there is a spinning wheel with a locked door to the left. Mario finds this key on a ledge to the lower-right of the spinning wheel, and uses it to open the door to the left and activate the wheel. Eventually Mario finds an area to use his airplane ability. The third key is in the room to the north of this area. Mario must use Mini-Yoshi to reach it speedily. This key opens the door on the right of the room after which Mario will encounter Sir Grodus.

There are eight Palace Keys of the second kind. They are placed in the rooms on the first two floors of Riddle Tower, and they can be used on the pedestals on the third floor. Mario and his partners must solve the puzzles to reveal the chests that contain the keys. On the third floor, Mario must use the Star Key first to unlock the pedestals, then place the Palace Keys on the pedestals to activate the planetarium, which will unlock the path to Shadow Queen's room in east Palace of Shadow.

The in-game descriptions read, "A key for a door in the Palace of Shadow." and "A key for a pedestal in the Palace of Shadow."