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As Eberron spins within The Material Plane, thirteen other planes, called The Outer Planes, spin in a complex orbit around it. The thirteen orbiting planes are separate from each other as they are from the Material Plane, but each of them occasionally becomes coterminous with the Material Plane, allowing direct connection between the two, but are otherwise either moving away from (waning), or moving toward (waxing) each other.
When coterminous, it is possible to move freely between the Material Plane and the coterminous plane at certain locations on each plane. For example, when Fernia is coterminous to the Material Plane, a character can travel to Fernia by means of a volcano or any extremely hot fire — and the natives of Fernia can enter Eberron just as easily. Also, when a plane is coterminous with the Material Plane, certain of its traits may bleed over into the Material Plane, or at least certain regions of the Material Plane, but not all planes have traits that spill over to the Material Plane.
As a plane draws close to becoming coterminous it is called waxing, and as it moves away after a period of being coterminous it is waning. There are no special effects on the Material Plane when a plane is waxing or waning.
When a plane is remote, it is difficult to establish any connection at all between it and the Material Plane.