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A Hidden Machine, HM for short (Japanese: ひでんマシン Secret Machine) is an item that, like a TM, is used to teach a Pokémon a move.
The difference between a TM and an HM is most pronounced in the earlier games in the series, where TMs were single-use items while HMs could be used an unlimited amount of times after they were obtained. HMs, unlike TMs, could not be sold for money, and the moves contained within could not be forgotten by Pokémon under normal circumstances. An HM move could be used by Pokémon even if they had fainted during battle.
All HM moves have the ability to be used outside of battle in the games they are HMs in, though a move's ability to be used outside of battle does not mean that it is necessarily an HM move. To be used outside of battle, specific Badges must be obtained by the player to advance the game's storyline.
In Generation I, five of the 165 moves are HM moves, and their use is essential for completing the game. Of these five, four of them remain as HM moves even in Generation V, while the fifth, Flash, retains the ability to be used outside of battle to light dark caves.
None of these moves are able to be forgotten in Generation I, and a Pokémon with these moves cannot be raised by the Pokémon Day-Care on Route 5. Unlike in later generations, all of these moves must be selected from the Pokémon screen to be used.
In Generation II two new moves become HMs. The five from Generation I return, while a new move, Whirlpool, becomes one, and an old move, Waterfall, becomes another.
A move deleter was added to the games, mostly with the intention of making possible the ability to delete Generation II moves that a Generation I Pokémon had learned in order to be able to trade it back, though with the side effect of HM moves now being able to be forgotten.
From this generation on, Pokémon with HM moves are allowed in the Day Care, and fathers with HM moves will pass the moves down to their children as they would a TM move. HM moves can also be used just by checking the obstacle that the HM will clear, such as surfable water or a movable rock.
In Hoenn one of the Generation II HMs lost its status, while another move became the eighth HM.
In Kanto, due to a lack of use for Dive, it was stripped of its HM status, though HM08 remains programmed into the game and can be hacked in to teach Dive. The first five HMs remain the same as Generation I, while HM06 and HM07 can be found in the Sevii Islands later in the game.
In Sinnoh, for the first time, one of the original HMs lost its status to a new move, while HM08 was brought back as a different move.
Defog lost its HM status in Johto for the return of Whirlpool; all other HMs retained their status from Sinnoh games.
HMs were reduced back to 6 in Generation V; HM07 and HM08 no longer exist in the coding of Pokémon Black and White. Dive returns for use in Unova. This is the first time that HM moves do not require Gym Badges for use outside of battle.
When a move is replaced by a HM, the HM move takes on the current PP of the replaced move. The same also applies for replacing old moves with new moves and TMs.
- In Generations I, II and V, all HMs only have one word in their names.
- Also, the only two HMs to date that have more than one word in their names are the only two to include "Rock" in their names despite not being Template:Type2.
- Ash's Grotle is the first Pokémon belonging to a main character of the anime to have used a current HM move, Rock Climb. Dawn's Piplup, however, is known for his Whirlpool, which did not become an HM again until long after he had first used it (though debatably, other Pokémon may have used Surf, Dive and Fly, though they were not directly ordered to use the move).
- The only current HM moves that are able to be learned by Pokémon by level-up are Fly, Waterfall, Dive and Whirlpool. Former HMs, such as Flash, are also learnable by level, but only since their loss of HM status.
- In Generation IV, all moves that were at one time an HM are learnable, with Flash available by TM70 and Dive available by move tutor. Defog and Whirlpool, which replace each other in the Sinnoh- and Johto-based games, are both HM05.
- HM moves have so far been only of the Flying-, Fighting-, Water-, and Template:Type2s.
- In HeartGold and SoulSilver Versions, on Route 20, a Trainer states "Surf is no longer the only HM move you use in water" when he is defeated. This is a reference to Generation I, where the only HM usable in the water was Surf.
- The only way to delete an HM move in the handheld games is to use the Move Deleter. HM moves can be replaced, however, in games where they are not HMs, such as Pokémon Stadium 2, Pokémon Colosseum and Pokémon XD, which allow any move to be replaced, Pokémon FireRed and LeafGreen and Pokémon Diamond, Pearl, and Platinum, which allow Dive, Flash, and Whirlpool to be forgotten, and Pokémon HeartGold and SoulSilver, which allow Defog, Dive, and Flash to be forgotten.
- The HM moves Cut, Fly, Surf and Strength have been in every main-series Pokémon game.
- Generation V is the first generation to have fewer HMs than the preceding Generation.
- The Mineral Badge of Johto is the only badge, prior to Generation V, which does not enable the use of an HM in any game it is present in.