In the Great Wheel cosmology, the Prime Material plane consisted of an infinite expanse filled with a fluid known as phlogiston, within which floated immense crystal spheres, each enclosing an entire system of one or more worlds. Specific conditions varied greatly from one crystal sphere to another, but they were all considered to be part of the same plane.
The mostly empty space contained inside the crystal spheres, known as wildspace, as well as all the worlds that existed within each sphere, were connected to the border of the Ethereal plane. It was unclear whether the phlogiston was also connected. The Ethereal plane served as a conduit between the Prime and the Inner Planes.
The Prime Material plane was also connected to the Astral Plane, which served as a conduit to the Outer Planes. There was no known direct way to travel between the Inner and the Outer Planes, or between the Astral and the Ethereal, that did not pass through the Material Plane first. Similarly, the Prime was the only plane in which spells that required a connection to the Ethereal or the Astral both worked.
Being the only plane that intersected both the Ethereal and the Astral, the Prime Material plane occupied an important position as a crossroads of the multiverse, and all other planes were defined in relation to the Prime.