Planejammer Campaign Setting
The Great Wheel
Known types of parallel universes include alternative universes, quantum universes, interpenetrating dimensions, parallel dimensions, alternative realities, alternative timelines, and dimensional planes, among others.
These dimensions can be pictured as soap bubbles floating through the murky insanity of the Far Realm in much the same way that Crystal Spheres float through the Phlogiston of the Prime Material Plane.
The laws of reality can vary wildly from one dimension to another. For example, there could be several dimensions the laws of physics and magic work like the ones in the Planescape Multiverse, with only slight differences, while in others either or both could be radically different if not absent. Our Earth and its universe are one dimension, and there are many parallels that “mirror” our earth such as the so called Gamma Terra and Dragonstar dimensions. (See also Multiple Oerths.)
Whether the number of dimensions is finite or infinite is a question for the sages, but there are certainly a multitude of them, making for a dazzling array of possibilities.
Probability travel is an extremely hazardous pastime, and one embraced by few. Indeed, few are even aware of the potential to do so.
Elminster of Shadowdale, Chosen of Mystra in Realmspace, has documented many trips to our own reality over the years. Murlynd of Greyspace spends much time in a world that resembles our own past in the days of the Old West.
It is not unknown, although the knowledge is rare.
The structure of the Multiverse is described as a web of 196,833 universes arranged in a pattern resembling a snowflake, each universe separated from its neighbors by a medium called the Bleed, the Far Realm, the Void and a host of other names. It is said that a “partial multiversal collapse” occurred in 1985, causing radical alterations to countless universes.