The Prime Material Plane is the plane in which Aldrazar and the surrounding cosmos exists. The phrase "Prime Material Plane," or simply "The Prime", is generally used by beings from the Realms of Ragna, and "prime" is used by the same beings to refer to the inhabitants of The Great Halo.

There is supposedly an infinite number of crystal spheres within the material plane, among them are the Crystal Sphere of Solus, Oerth, Mundus, Mystara, Rical and many more. However, in the cosmology shared by the Planescape and Spelljammer campaign settings, there is only one material plane where all the crystal spheres of many different worlds are separated by the Phlogiston.