Canon:Great Wheel

The Great Wheel cosmology is the standardized layout of the planes that was presented for the first time in Volume 1, Number 8 of The Dragon, released July 1977. It was presented again in the original (1st edition) AD&D Players Handbook, published in June 1978, and expanded upon in the original Manual of the Planes, released in 1987. It was the core cosmology in both editions of AD&D and the 3rd and 3.5 editions of D&D.

Many Outer Planes were renamed in Advanced Dungeons & Dragons Second Edition in the Planescape campaign setting, released in 1994. In the Third Edition Manual of the Planes, published in 2001, the old and new names were combined together, the Demiplane of Shadow was promoted to the Plane of Shadow, the Prime Material Plane was shortened to the Material Plane, and it was stated that each Material Plane is connected to its own unique Ethereal Plane.

The cosmology is usually presented as a series of concentric circles, with alternating spatial and transitive planes; from the center outwards, they are ordered as follows: Inner, Ethereal, Material, Astral, Outer Planes, and the Far Realm. The Shadow Plane and the Dimension of Time, if they are included, are separate from the others, and usually represented as being connected to the Material Plane. Demiplanes, although most commonly connected to the Ethereal Plane, can be found attached to any plane. All planes, save the demiplanes, are infinite in extent.

Inner Planes
The Inner Planes are made up of elemental matter and forces. They consist of the Elemental Planes and the Energy Planes.

Material Planes
The Material Planes are worlds that balance between the philosophical forces of the Outer Planes and the physical forces of the Inner Planes, these are the standard worlds of fantasy RPG campaigns. The Prime Material Plane is where the more 'normal' worlds exist, many of which resemble Earth. The 2nd edition Dungeon Master's Guide states there are several prime material planes, but several other 2nd edition products say there is only one Prime Material Plane rather than several.

Introduced in the Spelljammer setting, the Phlogiston is a part of the Material plane. It is a highly flammable gaseous medium in which crystal spheres holding various Prime Material solar systems float, traversable by Spelljammer ships.

Outer Planes
Alignment-based planes. The home of of gods, dead souls, and raw philosophy and belief.

Transitive planes
The transitive planes connects the other planes and generally contains little, if any, solid matter or native life.

Astral Plane
The Astral Plane is the plane of thought, memory, and psychic energy; it is where gods go when they die or are forgotten (or, most likely, both). It is a barren place with only rare bits of solid matter. The Astral Plane is unique in that it is infinitesimal instead of infinite; there is no space or time here, though both catch up with you when you leave. The souls of the newly dead from the Prime Material Plane pass through here on their way to the afterlife or Outer Planes.

The most common feature of the Astral Plane is the silver cords of travelers using an astral projection spell. These cords are the lifelines that keep travelers of the plane from becoming lost, stretching all the way back to the traveler's point of origin.

A god-isle is the immense petrified remains of a dead god that float on the Astral Plane, where githyanki and others often mine them for minerals and build communities on their stony surfaces. Tu'narath, the capital city of the githyanki, is built on the petrified corpse of a dead god known only as "The One in the Void." God-isles often have unusual effects on those nearby, including causing strange dreams of things that happened to the god when it was alive. God-isles are also the only locations on the Astral Plane that are known to possess gravity or normal time flows.

Ethereal Plane
The Ethereal is often likened to an ocean, but rather than water it is a sea of boundless possibility. It consists of two parts: the Border Ethereal which connects to the Inner and Prime Material planes, and the Deep Ethereal plane which acts as the incubator to many potential demiplanes and other proto-magical realms. From a Border Ethereal plane a traveler can see a misty greyscale version of the plane from which they are traveling; however, each plane is only connected to its own Border Ethereal, which means inter-planar travel necessitates entering the Deep Ethereal and then exiting into the destination plane's own Border Ethereal plane. Many demiplanes, such as that which houses the Ravenloft setting, can be found in the Deep Ethereal plane; most demiplanes are born here, and many fade back into nothingness here. Unlike the Astral Plane, in which solid objects can exist (though are extremely rare) anything and everything that goes to the Ethereal Plane becomes Ethereal. There is also something here called the Ether Cyclone that connects the Ethereal plane to the Astral Plane.

In the 3rd Edition, each Material Plane is attached to its own unique Ethereal Plane; use of the Deep Ethereal connecting these Ethereal Planes together is an optional rule.

Shadow Plane
The Plane of Shadow was a demiplane in an earlier edition of the game, but finally became a full-fledged transitive plane in the 3rd edition cosmology. It is, as one would expect, an empty plane of darkness, where shadows are cast without any source of light. The Shadow Plane connects to other planes not only through portals but also through darkness; this is especially true on the Material Plane, where a monster or mage might use the shadows as a form of transportation. It has also been speculated that the Shadow Plane connects many different cosmologies, thus making it possible for a planeswalker to travel between them. The d20 Modern campaign's Shadow Chasers, Urban Arcana, and the d20 Past campaign Shadow Stalkers, are based on this premise. In the Dungeons & Dragons Third Edition this is an optional rule.

Mirror Planes
Mirror planes were introduced in the Third Edition Manual of the Planes as an optional group of transitive planes. They are small planes that each connect to a group of mirrors that can be located in any other planes throughout the multiverse. A mirror plane takes the form of a long, winding corridor with the mirrors it attaches to hanging like windows along the walls. Mirror planes allow quick travel between the various mirrors that are linked to each, but each plane contains a mirror version of any traveler that enters it. This mirror version has an opposite alignment and will seek to slay his real self to take his place. All mirrors connect to a mirror plane, though each mirror plane usually has only five to twenty mirrors connecting to it.

Demiplanes
Demiplanes are minor planes, most of which are artificial. Demiplanes are commonly created by demigods and extremely powerful wizards and psions. Naturally occurring demiplanes are rare; most such demiplanes are actually fragments of other planes that have somehow split off from their parent plane. Demiplanes are often constructed to resemble the Material Plane, though a few &mdash; mostly those created by non-humans &mdash; are quite alien. Genesis, a 9th level arcane spell or psionic power, is one of the few printed methods for a player to create a demiplane.

The most notable demiplane is the Demiplane of Dread, the setting of Ravenloft.

Neth
Neth, the Plane That Lives, is a living, sentient plane of finite size that has an immense curiosity. The only access Neth has to the rest of the multiverse is through a single metallic, peach-colored pool on the Astral Plane. Those who look into the pool from the Astral Plane might notice a huge eye flash into focus on its surface, which quickly fades.

Sometimes, Neth will choose to encapsulate its visitors. Two folds of membrane will come together and ensnare and seal off the victims. Neth will then flood the compartment with either preservative or absorptive fluid. The preservative fluid will put the victim in temporal stasis, and the victim can be revived if the fluid is drained away. If the compartment is flooded with absorptive fluid, the victim will dissolve and be absorbed into Neth itself, including the victim’s memories.

Characteristics
Neth is living membrane the size of a continent. It is folded upon itself and resembles an enormous paper ball with a radius of about twenty-five miles. The spaces between the membrane’s folds can be a hand’s span across or larger than a city. The spaces are filled with air-saturated fluid, but visitors can still breathe in them. If the plane were spread flat, it would be about five hundred miles in diameter, and the average thickness would be approximately thirty feet.

Air- and water-breathing visitors to Neth can breathe and speak in Neth’s fluid normally. If they swim around unnoticed, they might see organ buds larger than city blocks, beings behind membranous capsules, and the humanoid antibodies of Neth mindlessly going about their business. Every part of the living demiplane has a soft, pink glow. Neth can also flush the fluid wherever it wants, carrying visitors along with it, usually to the Visage Wall to be questioned.

Gravity on Neth is the same strength as that on the material world; however, Neth chooses the direction of gravity’s pull and may change it at will. Time is normal on Neth. Neth can move its interior membrane at will, creating or destroying fluid-filled spaces.

Inhabitants
The only thing native to Neth is the plane itself. Neth creates humanoid subunits of itself called Neth’s Children, sometimes for specific short-term purposes before reabsorbing them.

Neth's Children are usually identical to flesh golems and resemble a vaguely humanoid mass of flesh. Though they are free-moving, Neth’s Children react to stimuli according to the preprogrammed will of Neth. Sometimes, Neth creates his children to serve as antibodies, but they are more often sent to other planes, instructed by Neth to explore and return for reabsorption, thus giving Neth more knowledge.

Locations
On Neth, the portal to the Astral Plane appears as twenty-foot-wide mouth-like cavity, which Neth can open and close at will.

At Neth’s center is a thick knot of membrane at least a mile across where all the folds come together. This serves as Neth’s brain. Other parts of the membrane also serve specific functions, which include areas where the membrane can be easily deformed for communication, encapsulation, and budding Neth’s Children.

The Visage Wall is an area of Neth's membrane where Neth communicates with visitors. It contains thousands of head-shaped bumps that resemble the likenesses of those previously absorbed by Neth. Neth speaks to its visitors from about five or six of the heads simultaneously, questioning them in order to learn more of the outside world.

Far Realm
The Far Realm is a plane far outside the others and often not included in the standard cosmology. It is inspired by the works of H.P. Lovecraft.

Temporal Plane
The Plane of Time was known as the Temporal Prime in the 1995 book Chronomancer. It is a plane where physical travel can result in time travel.

In 3rd edition products, some of the detail of Temporal Prime became incorporated into the "Temporal Energy Plane" mentioned in the 3rd edition Manual of the Planes. Dragon Magazine #353 associates it also with the "Demiplane of Time" that's appeared in various forms since 1st edition.