Prime Material Plane
The home plane of most mortals, this plane consists of an innumerable number of worlds, each located within its own crystal sphere. These crystal spheres float within the phlogiston, a flammable substance that fills the remainder of the space almost as an ocean, with currents allowing for ease of travel between certain spheres.
Travel from one sphere to another within the Prime is most often accomplished through the use of spelljammers; magical ships that use the power of a spellcaster to propel themselves through the void.
This plane is located in some sense in the center of the multiverse, laying coterminous to both the Astral and Ethereal Planes, allowing access to both the Outer and Inner Planes. It is here that most powers draw their power from, as most of their faithful make their kip on the Prime.
Dwellers from the Prime who travel on the planes are often referred to as "Clueless", due to the common misconceptions primes have about the planes, and the gape-mouthed expression many hold when traveling within them.