So you made it, eh? And now you think you're tough 'cause you can cast some spells and fly around on your silver cord? Well, I got news, you berk. I live out here and I was born tough. So why don't you just fly back home?
- Tavis the Reaver's greetings to an astral traveler in Sigil
Welcome, addle-cove. Welcome to the worlds beyond your world, the great wheel of the cosmos. This is a great place! Where else can a poor sod mingle with mighty minions of the great powers, sail the astral ocean, visit the flaming courts of the City of Brass, or even battle fiends on their home turf? Hey, welcome to the lands of the living and the dead! There's enough crusades, exploits, treasures, and mysteries to keep a band of adventurers busy for centuries to come (though why a body'd want to go to some of those places is beyond reasoning).
Before stepping through any door, a body'd better have a quick lesson in cosmology - how else is a basher going to know where and what things are? First, it's important to know just what a plane is. To the serious philosopher types, a plane's a world, or a collection of worlds, that operates according to its own particular laws, including those affecting magic, gravity, and even the morals of the place. On some of these planes, the laws of "up" and "down" aren't the same; on others, evocation magic yields different results; and elsewhere, behaving even slightly out of line with the powers of the place makes for grim results.
Planes are either immense and infinite, in which case they're just called planes, or they're limited by definite borders and are called demiplanes. The exact number of planes is unknown and probably infinite, and planar travelers know of only three main categories: The Prime Material Plane, the Inner Planes, and the Outer Planes. Still, those three have more than enough space for a flaming large number of different planes.
To get around in the planar multiverse, there are three basic rules to remember: the Center of The Multiverse, the Unity of the Rings, and the Rule of Threes. These truths pretty well describe the structure of all the universes, so learn them well!
The Center of the Multiverse
It's usually upsetting to Prime Material bashers when they hear that their little world isn't the center of the universe, which is why they're known to planars as the Clueless. Members of a faction called the Signers might argue otherwise, but smart folks say there's no particular center to the planar multiverse. Rather, it all depends on where you stand. Folks in Sigil see the City of Doors as the center of The Multiverse, folks on the Prime Material Plane say their own worlds are the center of the universe, and the efreet brag that the City of Brass is the center of it all. The thing is, maybe they're all right and maybe they're all wrong.
Maybe they're all right because - The Multiverse being infinite by most standards - no matter where you stand, that's the center of all things. The Signers have turned that idea into a whole philosophy: "I'm always at the center of The Multiverse; therefore, I must be the center of all universes," they say. 'Course, the Signer's aren't quite right, because by that logic everyone stands at the center of The Multiverse. (The Signers resolve this little paradox by ignoring it.)
In blunt words, the fact is that there ain't any place in the whole Multiverse that's more important than any other. For instance, Mystara on the Prime Material Plane is not the most powerful, influential, or important point of The Multiverse; it's not the sole reason all the other planes and powers exist. Hey, the uncounted layers of the Abyss stink of evil itself, but exactly zero of the other Outer Planes kowtow to them, regardless of what the fiends there claim.
Some places - like Sigil - are more useful than others, though. Just because it's not the center of the universe, don't think it ain't important, berk.
The Unity of Rings
A ring's a thing without a beginning or end. Remember that, because rings are the second key to understanding the planes. Everything comes in rings. Sigil's a ring, the Outlands are many rings, the Outer Planes form a ring, the Elemental Planes form a ring - this is the way of The Multiverse, understand?
On the Outer Planes, the Great Road is the band of the ring, and all the planes are its gemstones. Following the Great Road, Mechanus leads to Acheron, Acheron leads to Baator, Baator leads to Gehenna, and so on. By following the road, the order never changes.
On the side, the powers think in rings, too - circles upon circles of logic that never go anywhere. A body's always got to watch out for their endless snares.
The Rule of Threes
"Good things come in threes," they say. Well, so do bad things. Either way, the number 3's important - some say it's got power. Things out here tend to happen in threes, like Prime Material, Inner, and Outer Planes; Good, Evil, and Neutrality; Law, Chaos, and Neutrality; even prime, planar, and petitioner. See two things and ask, "Where's the third?"