Sinnoh is effectively split into 5 sub-regions:
- Western Sinnoh. This is where you start the game and contains about half of the towns and cities in the game, although clearly they will not be the most important ones.
- Sinnoh Mountain Range. This area is right in the middle of Sinnoh and connects all the areas together. The main feature here is a dungeon called Mount Coronet. I have been told that this dungeon is the largest in any Pokémon game so far, and I am really excited about exploring it. Not only that but it slowly unlocks as you get more and more HMs. Contrary to implications by various websites, Mount Coronet is not the only thing in the mountain range; various routes pass through here, and the town of Celestic is also located here.
- Eastern Sinnoh. This is obviously the other side of the mountain range and roughly contains the other half of the towns and cities; in particular the league headquarters are here.
- Northern Sinnoh. This area contains Snowpoint City and the routes leading up to it, covered in snow. It also contains an island called the Battle Zone, which contains the Sinnoh Battle Frontier among other things.
- The Underground. While traversing the Underground will not make you any progress in the overground, it is still an area for exploration, and is split into 4 distinct parts. You get to the Underground by digging using a particular Kit in the game.