From Bulbapedia, the community-driven Pokémon encyclopedia.
Key Items (Japanese: 大切なもの indispensable items) are special items that players can only obtain once, and either aid the progression of the storyline or allow access to new areas. They rarely have anything to do with the player's Pokémon, and are mostly specific to the different games. The items cannot be bought or sold at Poké Marts, disposed of, or given to Pokémon to hold. Since Generation II, these items have been given their own classification, and have been stored in a separate pocket in the Bag. Key Items can also be registered to a button for use in the overworld.
Obtaining Key Items
Key Items cannot be sold.
Key Items are generally not found lying on the ground, but are usually obtained throughout the storyline, and are given to the player by other people. Once a player has obtained a Key Item, there is no way of getting rid of it, unless it is part of the main storyline. For example, the Machine Part found in Kanto in Generation II as part of the mission to save the Kanto Power Plant could only be removed when the player had taken it back to the Plant and given it to the manager. Some Key Items, such as the SquirtBottle, are not deleted once their usefulness has passed, and remain in the inventory for the remainder of the game. Prior to Generation IV, Key Items could be deposited into a PC.
Common Key Items
Although many Key Items are specific to the version of the game being played, basic items continue to feature:
Exchanging Key Items
The exchanging of one Key Item for another has been present since Generation I: a Bike Voucher was given to the player to exchange for a Bicycle. Key Item exchanging can only happen once, with the exception of Generation III, in which the player can swap the two bikes (the Mach Bike and the Acro Bike) as they choose. In any case, only one or two Key Items can be exchanged.
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