Planescape

The planes of Osidria are arranged in stacked layers, with each layer containing multiple planes that may bleed into each other at points.

The Prime Material Plane
Also referred to as Osidria, the plane inhabited by most mortals, the Prime Material is the centermost plane, easily accessed from most anywhere in the Planescape. It is the ultimate buffer - the opposing elemental planes are kept separate, and celestials and fiends cannot wage open war without risking the annihilation of the Prime Material.

The Elemental Planes
Planes which embody one of the four elements. They bleed into the Prime Material plane in areas which can be said to epitomize their element: for example, a wildfire might spontaneously open a gateway to the Elemental Plane of Fire, or a huge, pristine crystal might double as a portal to the Elemental Plane of Earth.

The Elemental Plane of Fire
A plane of stunning beauty and terrible perils, the Elemental Plane of Fire is in a state of perpetual conflagration. The plane plays host to a great war between the Efreeti, who live as a civilization in a great metropolis built with the use of slave labor, and the Salamander tribes, proud nobles who reject the rule of all but themselves.

A portal to the Elemental Plane of Fire is known to exist somewhere in the interior of the island of Matao, but its physical location has not yet been discovered.

The opposing plane to the Elemental Plane of Fire is the Elemental Plane of Water.

The Elemental Plane of Earth
The opposing plane to the Elemental Plane of Earth is the Elemental Plane of Air.

The Elemental Plane of Water
The opposing plane to the Elemental Plane of Water is the Elemental Plane of Fire.

The Elemental Plane of Air
An elemental plane where gravity is relative and civilization exists in tangled cities built on clouds. It is ruled by the Djinn from the Cloud Citadel, where the djinn sultana rules. The opposing plane to the Elemental Plane of Air is the Elemental Plane of Earth.

The Paraelemental Planes
New planes form at the intersections of the elemental planes, forming new planes that combine the two. These planes are Magma, Smoke, Ice, and Ooze.

The Upper Planes
These planes were once paradises for good creatures. Now, they are ruined and defunct, and any inhabitants that might remain have refrained from contact with the Prime Material.

The Lower Planes
Also called the Eight Hells, these eight planes are inhabited by fiends and each is ruled by an archdevil. They are considered to be equal, though each one is locked in a constant power struggle with the other seven.

The Celestial Planes
The two celestial planes are the Solar Plane, inhabited by Niphu, and the Lunar Plane, inhabited by the other major deities. They are located above the Upper Planes.

The Outer Plane
A scary place where terrifying creatures swim through the partially liquid air. It is in the outer limits of the Outer Plane that a vast multitude of Great Old Ones were sealed in great celestial prisons. These prisons emit huge amounts of energy to keep the GOO in stasis, and can be seen all the way from the Prime Material, where they are called "stars".