Transitive Planes

Transitive Planes is a collective term used to describe those planes of existence that connected other planes together, or could be used as a means of travel between points on a single plane. The various cosmology models each described them differently and ascribed them slightly different traits, but all were accessed by either magic spells, spell-like abilities, or portals. The known Transitive Planes were:
 * The Astral Plane, which later became the Astral Sea
 * The Ethereal Plane, which was destroyed by the Spellplague
 * The Plane of Shadow, which later became the Shadowfell
 * The Spirit World, which only connected to Kara-Tur
 * The Plane of Mirrors, which connected to the back of reflective surfaces

Each had their own unique structure and means of travel, and each connected a different set of planes together.