Quick Answer: The planes of the Dungeons & Dragons roleplaying game constitute the multiverse in which the game takes place. In the earliest versions of Dungeons & Dragons, the concept of the Inner, Ethereal, Prime Material, Astral, and Outer Planes was introduced; at the time there were only four Inner Planes and no set number of Outer Planes. This later evolved into the Great Wheel cosmology. The fourth edition of the game used a different, very simplified cosmology with just six main planes called the World Axis Cosmology. The fifth edition has brought back a new version of the Great Wheel cosmology. 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 seventeen planes in total, most of which are unique to Eberron.