From the Super Mario Wiki
Flipside is an interdimensional town in Super Paper Mario, created by the Ancients. Flipside is odd and irregular compared to most places in the Mario series, introducing a whole new style of its own. There is a mirrored version of Flipside called Flopside. The people of Flipside are pixel-like and have bodies made of separate layers. In one place of Flipside, a chef named Saffron owns and runs Sweet Smiles. Flipside is the home of some very important characters, including Merlon, Howzit the item shop owner, Merluvlee the fortune teller and many more.
Flipside has three different "floors" which can be accessed by elevators interconnected with each other. They are numbered sequentially in the north American version, but in the PAL version, the first floor is known as the "Ground Floor", the second floor becomes the "first floor", and the third floor becomes the "second floor". Almost every structure in Flipside, except a few places and roofs, is whitish or gray with a hint of yellow, and the sky of Flipside is also whitish-yellow, with many geometric cubes and symbols hovering about. There is an area outside of the town of Flipside, simply called the Flipside Outskirts, which seems to be a strange, interdimensional wasteland filled with monsters. Flipside also houses a Pit of 100 Trials, and both Flipside and Flopside are depicted on the walls of the first 25 rooms, paired with Mario's first Pure Heart.
Within Flipside, there exist several structures called Heart Pillars. Mario must place the Pure Hearts that he finds throughout his adventure into these structures to open up a path to the next world and the next Pure Heart. Flipside Tower, located in the middle of the town, controls the flow of dimensional energy to teleport people to other worlds. Heart Pillars and a different Tower are also found in Flopside.
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