Lord Zack's Planescape
the Great Wheel
The Great Wheel is the name of one of the most popular models of the planes, especially in the city of Sigil. As the name suggests the theory holds that the planes are arranged in a circular manner, the Inner (also Elemental or Fundamental planes) arranged around the Ethereal Plane and the Outer or Spiritual Planes arranged around the Outlands, the plane of Neutrality, according to they’re alignment.
Three principles are said to govern the universe, these are the Rule of Threes, the Unity of Rings and the Center of All. These state that things tend to come in threes like the varieties of planes, material, fundamental, and spiritual, things tend to be circular, for instance you can go from one plane and by traveling through each of the planes in turn, return to where you started, and where ever you are is the center of everything. These are used as heuristics for the study of the planes,and are meant philosophically as much as they are as geographically, though it is hard to separate the two on the Outer Planes where the theory has it’s most proponents. The principle of Center of All is sometimes used to challenge the theory, as some believe that if the planes were a Wheel they’d have to have a single center.