The Inverse Material Plane

The inverse material plane is an alternate dimension uniquely tied to The Unnamed Land, and is believed to have been created by the same event which seeded life and magic in the prime material plane.

The differences between the Inverse Plane and Prime Plane may seem random or even arbitrary, but this is an observation based on a flawed concept. Rather than considering the Inverse Plane to be an alternate version of our own Prime Plane, it is much more likely that the two planes were created simultaneously with different sets of parameters.