The Cosmic Wheel was a concept proposed by scholars regarding the structure of all planes, as well as all planar magic. By far the most commonly used model of cosmology to describe the known world, it was composed of three fundamental anchors: the Upper Realms, Lower Realms, and the numerous "Inner Planes." These were supported and fed by the Elemental Realms, which bled into each other to create sub-planes. Surrounding these inner planes were a sea of energy in the Positive Border and Negative Border; some scholars considered these energies to be Elemental themselves, but that philosophy was much-debated.
The Inner Planes were connected to the others by a "transitive" plane, the Astral Border plane, a mostly-empty sea of silver nothingness. Additionally, all planes had a transitive layer called the Ethereal Border, barely lifted from the planes themselves, where Undead and other spirits could be trapped. To travel between the planes, creatures had to navigate these dangerous places.
In the center of the Cosmic Wheel was the Material Realm plane, the place where all aspects of the planes converged. Echoes of the Material existed in the Vivid Realms and Umbral Realms, which respectively heightened and dampened both emotions and magic.