Physical/special split — the move set revolution.
Diamond, Pearl, Platinum (and the Brilliant Diamond / Shining Pearl remakes) · Released 2006
Sinnoh is the Gen 4 region, and the home of the most impactful balance change in Pokémon's 25-year history: the physical/special split. Before Gen 4, move damage type was determined by the move's *elemental* type — all Fire moves were special, all Rock moves were physical, regardless of what made sense. Gen 4 split this per move, meaning Crunch became physical, Ice Beam stayed special, and Dragon Claw finally existed as a physical Dragon option.
Sinnoh is based on Hokkaido, Japan's northernmost island — which is why Mt. Coronet runs north-to-south through the middle of the map and why the region has the coldest, snowiest routes in the franchise. Dialga and Palkia represent time and space, directly referencing Ainu creation mythology from Hokkaido's indigenous culture.
Diamond & Pearl's original Sinnoh soundtrack is considered by many fans to be the best in the franchise — composer Junichi Masuda wrote the route themes using a 'walking tempo' deliberately matched to the player's overworld walking speed.