Cosmology was the study of the cosmos—the structure, organization and composition of all that existed. What most people would call the "known universe" was labeled the Prime Material Plane. It held the world of Toril, the moon, the sun, and extended out into space to the stars. How far out into space was a question of cosmology, and the answer might depend on which cosmological model was used to describe the Prime Material plane. But there were many other planes of existence, some very much like the Prime and some so alien that they almost defied description. Like other dimensions, these other planes might overlap, surround, or penetrate the Prime Material plane, or they might be connected by some mystic avenue, often requiring magic to explore. Together, all the various planes were usually referred to as the "multiverse" and they were the focus of cosmologists as they tried to map, measure, and understand the planes and their relationship to life, to death, and to each other.
There have been numerous theories on the structure of the multiverse over the ages. The three major ones are as follows:
- World Axis cosmology: The post-Spellplague model.
- World Tree cosmology: The most commonly accepted pre-Spellplague Multiversal structure.
- Great Wheel cosmology: A popular alternative to the World Tree theory.
These models are simply that—models. They were theoretical ideas to make sense of the strange reality of the multiverse by mortal sages.
The Prime Material Plane
At first it was thought that there was an infinite number of material planes, among them the Prime Material plane within which Toril resided, the Abeiran material plane, Oerth's material plane, Krynn's material plane, and the universe in which Earth was located.
As the existence of other worlds started to be confirmed, each of the cosmological models had a way of explaining how they connected, related, or coexisted with one another. Of course, the scholarly inhabitants of each world thought that theirs was the Prime Material plane, but even early cosmological models showed that this was just a point of view. Other Material planes discovered in this early description included:
- Abeiran Material Plane
- Although Abeir once existed in the same cosmology as Toril, Ao split it into a cosmological twin of the local multiverse at the time, with its own separate parallel and fundamental planes. Other than the fact that it existed and that it occasionally interacted with the Prime, little was known of it.
- Oerth Material Plane
- A distant world visited by Khelben Arunsun the Younger, Oerth differed enormously from Toril, being a geocentric world around which its sun and planets rotated.
- Terran Material Plane
- A world called Earth was located there, of which little was known, but which was once visited by Elminster in the Year of the Prince, wherein he met with a scholar there named Ed of the Greenwood.
However, by the mid-14th century DR, it was generally accepted that there was only one material plane (called the Prime Material plane), which contained the crystal spheres of many different worlds separated by the phlogiston.
Though most planes were described by one or more of the cosmological models, there were some exceptions that did not have a clear place in any known cosmology. These planes existed in unknown, nigh unreachable places and only a few well-informed scholars even knew of their existence. These planes might drift distantly in one of the fundamental planes or lie entirely outside of the World Axis or any of the other models. These planes often did not follow the same physical laws as did the other planes and were immensely inhospitable.
The known anomalous planes were as follows, though undoubtedly more existed:
- Far Realm
- The Far Realm was a distant and terrifying plane that lay "outside" of the rest of creation. This maddening realm was feared for its power to twist unfortunate visitors into gruesome monsters and it was from here that aberrations came.
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Prime Material plane • Feywild • Shadowfell • Fugue Plane
Transitive Planes: Astral Plane • Ethereal plane
Inner Planes: Elemental Plane of Air • Elemental Plane of Water • Elemental Plane of Earth • Elemental Plane of Fire • Elemental Chaos
Para-Elemental Planes: Frostfell • Swamp of Oblivion • Fountains of Creation • Great Conflagration
Quasi-Elemental Planes: Lightning • Radiance • Minerals • Steam • Vacuum • Ash • Dust • Salt
Outer Planes: Arcadia • Mount Celestia • Bytopia • Elysium • Beastlands • Arborea • Ysgard • Limbo •
Pandemonium • Abyss (Layers) • Carceri • Hades • Gehenna • Nine Hells • Acheron • Mechanus
Energy planes: Positive Energy plane • Negative Energy plane
Planar Pathways: Infinite Staircase • Oceanus • Mount Olympus • Styx • Yggdrasil
Prime Material plane • Cynosure • Fugue Plane
Transitive Planes: Astral Plane • Ethereal plane • Plane of Shadow • Spirit World
Celestial Outer Planes: Arvandor • Brightwater • Dwarfhome • Dweomerheart • Gates of the Moon • Golden Hills • Green Fields • House of Knowledge • House of the Triad
Fiendish Outer Planes: Abyss (Layers) • Barrens of Doom and Despair • Blood Rift • Clangor • Deep Caverns • Demonweb Pits • Fated Depths • Fury's Heart • Hammergrim • Nine Hells • Nishrek • Supreme Throne
Neutral Outer Planes: Dragon Eyrie • Heliopolis • House of Nature • Jotunheim • Warrior's Rest
Inner Planes: Elemental Plane of Air • Elemental Plane of Earth • Elemental Plane of Fire • Elemental Plane of Water • Positive Energy plane • Negative Energy plane
Planar Pathways: Infinite Staircase • River of Blood • World Tree
Prime Material plane
Fundamental planes: Astral Sea • Elemental Chaos
Astral dominions: Arvandor • Banehold • Celestia • Cynosure • Deep Wilds • Demonweb Pits • Dismal Caverns • Dwarfhome • Eternal Sun • Fugue Plane • Gates of the Moon • Green Fields • House of Knowledge • Nine Hells • Nishrek • Supreme Throne • Towers of Night • Tu'narath • Warrior's Rest
Elemental realms: Abyss (Layers) • City of Brass • Cresting Spires • Fimbulwinter • Hidden Realm • Muspelheim • Root Hold • Sky Home • Steading • Thraotor • Undying Pyre • Zerthadlun
Parallel planes: Feywild • Shadowfell
Anomalous planes: Far Realm