HMs are multiple use TMs that serve a purpose out of battle, as well as being moves for in-battle use; however, prior to Generation V, in order to use an HM's purpose out of battle, a Gym Badge is required. HM05 is:
In Diamond, Pearl, and Platinum, HM05 has the item sprite of a Template:Type2 HM, even though Defog is a Template:Type2 move. It is possible that this is because Defog's HeartGold and SoulSilver counterpart, Whirlpool, is a Water-type move.
HM05 is the only HM and only machine move so far that has changed moves within a generation, from Defog in Diamond, Pearl, and Platinum to Whirlpool in HeartGold and SoulSilver.
HM05 has contained the most different moves of any HM, teaching four different moves throughout the series. Due to its changing moves mid-generation as mentioned above, it is also the only machine that has contained four different moves (the maximum possible) that contained the same move for more than one generation.
Interestingly, two of the moves it has contained are the two HM moves which first appeared as HMs in Generation II.
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