Plane

A plane is a particular subdivision of a universe with consistent properties. Many planes are finite, although some are infinite. Most universes have a variety of planes, but technically a universe could have as few as one (as seems to be the case with the bizarre universe called Earth).

Planar Affiliation
The vast majority of planes have an affiliation with a particular abstract construct, such as an alignment or an element, but each universe has one unique plane which is unaffiliated with any particular construct.

There are several categories of planes based on affiliation:
 * Most universes, by default, have exactly one unique unaffiliated plane which contains the universe's world. This plane is referred to by any of a number of names, including "World Plane," "Material Plane," and "Prime Materials Plane," among others. The world plane can always support sapient life. (Note that the universe Corvus is unique in that it has three world planes instead of one.)
 * Elemental planes, which are affiliated with one (or possibly more) of the elements. Elemental planes are almost always hostile to sentient life, and tend to be inner planes.
 * Alignment planes, which are affiliated (strongly or weakly) with one of the nine alignments. Alignment planes tend to be outer planes, and are almost always the planes where the gods of the universe reside (typically, but not always, on the plane affiliated with their own alignment). If a god resides there, the plane is sometimes referred to as a "dominion plane". Alignment planes may or may not be hostile to sentient life depending on the plane; good- and law-aligned planes tend to support life, whereas evil- and chaos-aligned planes tend to be hostile, although there are always exceptions. Note that souls are always supported on their affiliated plane.
 * Energy planes, which are affiliated with a polarity of energy -- most commonly either positive or negative, although some individual planes may have non-binary poles of energy. All energy planes, even positive energy planes, are hostile to sentient life, because even the raw energy of life is lethal when you're swimming in it. Note that energy planes are occasionally (and ambiguously) referred to as "material planes," not to be confused with the world plane, above, which can also be referred to as "the material plane."
 * Void planes, which are huge, often unaffiliated planes which support no life at all. Void planes are almost always infinite, and tend to be coterminous with several other planes (see Planar Boundaries, below). This makes them useful as a medium for travelling between planes.
 * Mirror planes, which lie alongside another plane (often, but not always, the world plane). Mirror planes are coterminous with their host plane at all points. The mirror plane is usually hostile to sentient life if and only if the host plane was, but occasionally a mirror plane can be hostile when the host plane supported life, or vice versa. A mirror plane usually appears to duplicate its host at every point, except twisted by whatever construct the mirror is affiliated with. If the mirror plane is unaffiliated, its duplicate image may appear twisted in simply a bizarre way (such as having inverted colors, or appearing foggy), or may be an absolute perfect duplicate to every detail. Although mirror planes are always coterminous with their host plane, they need not be coterminous to the same set of other planes, thus potentially making them useful as a travelling medium. If the mirror plane has differing laws of physics from the host plane, it might also be used to facilitate travel across the host plane itself.

Planar Boundaries
All planes connect to at least one other plane at some point. These points are called "boundaries." When two planes share a boundary, they are called "coterminous." Each plane in a universe is connected, directly or indirectly, to every other plane in that universe. No plane is ever coterminous with a plane in a different universe. Even infinite planes can have boundaries: for example, an infinite plane may only be finite in one direction, and come to a boundary on one side; an infinite plane is always coterminous with any plane it completely envelops; and in general, planes are not bound by three-dimensional space, so any point in an infinite void may be a "boundary" which is coterminous with any point in another plane.