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The planes of the Dungeons & Dragons roleplaying game constitute the multiverse in which the game takes place. Each plane is a universe with its own rules with regard to gravity, geography, magic and morality. There have been various official cosmologies over the course of the different editions of the game; these cosmologies describe the structure of the standard Dungeons & Dragons multiverse.
The concept of the Inner, Ethereal, Prime Material, Astral, and Outer Planes was introduced in the earliest versions of Dungeons & Dragons; at the time there were only four Inner Planes and no set number of Outer Planes. This later evolved into what became known as the Great Wheel cosmology. The 4th Edition of the game shifted to the World Axis cosmology. The 5th Edition brought back a new version of the Great Wheel cosmology which includes aspects of World Axis model.
In addition, some Dungeons & Dragons settings have cosmologies that are very different from the "standard" ones discussed here. For example, the Eberron setting has only thirteen planes, all of which are unique to Eberron.