Flipside is a small city in which the events of Super Paper Mario occurred. Flipside exists outside other dimensions and is highly geometric in design. Flipside has a sister city, Flopside that very few know about, with the exception of Merlon and Mario. Similarly, the citizens of Flopside know little to nothing about Flipside.
Like Toad Town and Rogueport before it, Flipside and its counterpart Flopside act as the central hub of the game, with each world or chapter spawning off from it. Whenever Mario obtains a new Pure Heart, he can access another world from Flipside (or Flopside, depending on which Pure Heart) and unlock more on the two cities. There are many shops and friendly people here. There are also many secrets in Flipside, some of which are necessary to progress.