Great Wheel

Description
The "Great Wheel" is the name for the collection of planes (multiverse) that make up the D&D Cosmology.

Planar Interactions
By definition, planes are infinite or near-infinite expanses, whether they are flat worlds, layered vaults or spheres hanging in space. So the idea of planes "interacting" is difficult to fathom. As a metaphor, imagine the various planes floating near each other in a three-dimensional constellation or cluster. They are not necessarily "above" or "below" each other, though there is a social tendency to call the good-aligned planes of the the Outer Planes the "upper" planes, and the evil-aligned one the "lower" planes. What does matter is if two planes are separate, coterminous or coexistent.
 * Separate Planes: Two planes that are separate do not overlap or directly connect to one another. They are like planets in different orbits. An Outer Plane, for example, has no direct connection with the Material Plane. The two planes are separate, and the only way to travel from one to the other is through a third plane, such as the Astral Plane, or the use of a planar portal.
 * Coterminous Planes: Planes that link together at specific points are coterminous. Think of them as touching one another. Where they touch, a traveler can leave one reality and enter another. The Astral Plane, again, is coterminous with many other planes as it exists alongside them and can be accessed from them.
 * Coexistent Planes: If a link between two planes can be created at any point, the two planes are coexistent. These planes overlap each other completely. As an example, the Ethereal Plane is coexistent with the Material Plane. With the right magic, items or rituals, inhabitants of the Material Plane can see and interact with those on the Ethereal Plane, and vice versa.

The D&D Cosmology
Also known as the Great Wheel (see photo above), it consists of an innumerable number of planes, but the most important are the following thirty-nine.
 * Material Plane: The Material Plane refers to the collection of Material Planes that exist, including settings such as Oerth, Athas, Toril, Eberron and Greyhawk.
 * Three Transitive Planes: The Astral Plane, Ethereal Plane and Plane of Shadow all act as conduits for interplanar travel.
 * Eighteen Inner Planes:
 * Four elemental planes (Air, Earth, Fire and Water)
 * Two energy planes (Positive and Negative)
 * Four paraelemental planes (where two elements meet; Ice, Ooze, Magma and Smoke)
 * Four positive quasielemental planes (where an element and positive energy meet; Lightning, Steam, Mineral and Radiance)
 * Four negative quasielemental planes (where an element and negative energy meet; Vacuum, Salt, Dust and Ash)

The Material Planes sit in the center of this arrangement.
 * Seventeen Outer Planes:
 * Heroic Domains of Ysgard
 * Ever-Changing Chaos of Limbo
 * Windswept Depths of Pandemonium
 * Infinite Layers of the Abyss
 * Tarterian Depths of Carceri
 * Gray Wastes of Hades
 * Bleak Eternity of Gehenna
 * Nine Hells of Baator
 * Infernal Battlefield of Acheron
 * Clockwork Nirvana of Mechanus
 * Peaceable Kingdoms of Arcadia
 * Seven Mounting Heavens of Celestia
 * Twin Paradises of Bytopia
 * Blessed Fields of Elysium
 * Wilderness of the Beastlands
 * Olympian Glades of Arborea
 * Concordant Domain of the Outlands
 * Demiplanes: An unknown number exist throughout the Multiverse.

The Plane of Shadow and the Ethereal Plane are coexistent with the Material Planes. All planes are coexistent with the Astral Plane, which envelops the whole cosmology like a cloud.

The Inner Planes surround the Material Plane. They are separate from the Material Plane, though coterminous with one another.

The Outer Planes are arranged in a great wheel around the Material Plane. Each Outer Plane is coterminous with the planes on either side of it, but separate from the others. The exception is the Concordant Domain of the Outlands, which is coterminous to every other Outer Plane and thus acts as a central hub for dealings among outsiders.

The Outer Planes are coexistent with the Astral Plane. They are separate from the Ethereal Plane and the Plane of Shadow. The Outer Planes are made up of related layers, and the most common access is through the top layer of each plane. The good-aligned planes, known as the celestial planes, are linked by the path of River Oceanus, while the evil-aligned, infernal planes are linked by the path of the River Styx.

There are a large number of finite demiplanes that connect all over the place. Individual conduits, freestanding gates and vortices can also be found.