A plate is a hold item that can be given to Pokémon introduced in Generation IV. There is a plate for every type except . All of the plates power-up moves of that specific type, and you can also give Arceus the plate to change his type. Depending on which plate Arceus is holding, the move Judgment changes type.
In Pokémon Diamond, Pearl, and Platinum, you may find these mystical plates Underground or in Amity Square if you talk to your Pokémon once in a while. You can also find one of each around Sinnoh with an inscription about the creation of Pokémon.
In Pokémon Black and White, every plate can be found in the Abyssal Ruins, with the exceptions of the Draco and Splash Plate. The Draco Plate and Splash Plate can be given in Route 13 by speaking to a trainer in a building.