HM
A Hidden Machine, or HM for short (Japanese: ひでんマシン Secret Machine), is a machine used by Pokémon Trainers to teach a Pokémon a new technique, or move, that it would not otherwise learn. HMs can be used over and over again on many different Pokémon, unlike Technical Machines, which are good for one use only. All HM moves are usable out of battle, and most are essential for completing the game.
To use an HM out of battle, the Pokémon party menu can be viewed, and under the HM-wielding Pokémon's options, the HM move it knows will be displayed along with the options, and it may be selected to trigger the HM's on-field use. In Generation II and beyond, this can simply be done by pressing A at the selected field obstacle and confirming to use it.
Unlike all other moves, HMs cannot be forgotten without the aid of the Move Deleter. The reason for this is a player can be stuck on the wrong side of a barrier, for example a smashable rock, without the appropriate HM move. In addition, Pokémon with HM moves cannot be transferred from a Generation III game to a Generation IV game via Pal Park, and so the HM move must be deleted before the transfer can take place, meaning that some special event Pokémon could not be migrated.
In Generation IV, if a player tries to release the only Pokémon they have that knows certain HM move, the Pokémon will come back, with the message, "<name> came back! Was it worried about you?" However, the game does not prevent the player from deleting the moves of such Pokémon.
A mutually compatible HM move be passed down through breeding with the male Pokémon passing down moves in the egg groups, much like mutually compatible TMs can be. As of Generation IV, only five Pokémon learn a HM move through leveling up. Strangely though, if a Pokemon learns an HM move through level up (for example Goldeen with Waterfall), the game will only let you delete the move with the move deleter, even though the Pokemon did not learn the move via the HM.
While most HM moves are necessary to complete the games that they are HMs in, many of them lack usefulness in Pokémon battles. Thus, players often capture extra Pokémon to keep in their parties solely to use HM moves.
Each generation has introduced new HMs or replaced old ones that have no out-of-battle use in the new region, such as Whirlpool in Hoenn. The original five HMs remained the same from Generation I up to Generation III, when Flash was replaced with Defog. Some moves that lost HM status were later available by TM or move tutor.
All HM moves require a certain badge to be acquired in order to be used outside of battle. This is so that even if a player gets a Pokémon with HM moves early, they cannot use them to change the sequential order of events in the games.
The English name, Hidden Machine, is a pun on the Japanese name, ひでんマシン (Hiden Mashin). Hiden (秘伝) means "secret".
For their in-game locations, see List of TM and HM locations.
