From Bulbapedia, the community-driven Pokémon encyclopedia.
Key items are special items that players can only obtain once, that either aid the progression of the storyline, travelling or the access 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 or disposed of. Since Generation II, these items have been given their own classification and have been stored in a seperate pocket in the bag.
Obtaining key items
Key items are not generally found lying around on the ground. They are usually obtained throughout the storyline and are given to the player by other people. Key Items cannot be disposed, bought, sold or held by Pokémon. 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 item Machine Part found in Kanto in Generation II as part of the mission to save the Power Plant could only be removed when the player had taken it back to the Plant and given it to the manager.
Common key items
Although many Key Items are specific to the version of the game being played, basic items continue to feature.
- Bike - An item that enables players to travel faster than walking.
- Rod - An item that enables players to fish for Pokémon in lakes or the sea.
- Keys and cards - Items that enable players to open doors and access new areas, such as the Card Key.
- Tickets - Items that enable players to board ships, enter contests and exchange for other key items.
Exchanging key items
The exchanging of one key item for another has been present since Generation I: a Bike Voucher, a seemingly useless item, was given to the player to exchange for a Bike. Key item exchanging can only happen once, with the exception of Pokémon Ruby, Sapphire and Emerald, 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 items could be exchanged.