HMs are 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 Water-type HM, even though Defog is a Flying-type move. It is possible that a Water-type HM was considered for the position before Defog was finalized. Whirlpool, a Water-type move itself, takes the spot in HeartGold and SoulSilver, though this is likely unrelated, as HeartGold and SoulSilver came out more than three years after Diamond and Pearl were released.
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 at present) 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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