The Outer Planes
The outer planes are also called the Planes of Power since they are the homes of the most powerful extraplanar beings in the known planes of existence. They have a mixture of elements that supports a rich diversity of life, often (but not always) similar to life on the Prime Material planes. It is unknown whether this is because the inhabitants of the outer planes affect the creatures on the Primes, or because the Prime planes impose order on the outer planes.
The conventional view of the outer planes is of a great wheel divided into 16 pieces. Each piece is the upper layer of a plane and is joined to the upper layers of the two planes adjacent to it. Planes have an variable number of planar layers, also called levels. in the center of the wheel, joined to it by the Astral plane, are the Prime Material, Ethereal, and inner planes.
The above is accurate as far as it goes, but things are more complicated than that. Each planar layer is physically infinite in the same manner as the Prime Material, inner, and Ethereal planes. The Astral plane coexists with the first (uppermost, closest to the Astral) layer of each plane. but there are also conduits that reach the deeper layers, enabling rapid movement of beings and great powers from one plane to another. Each layer of the outer planes contains a number of different realms ruled by various Powers or forces.
It has also been revealed that there is another outer plane that does not fit -
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