This here is a basic overview of those planes of primary
importance to Sigil...
The Inner Planes.
These planes are also called the elemental planes, because they consist of the
elements, like the Plane of Earth or the Plane of Fire.
The Outer Planes.
These planes reflect moral, psychological elements rather than physical
elements. Good and evil, law and chaos. In the middle are the Outlands, a
neutral plane. When on the outlands, the closer one is to a certain outer plane,
the more the Outlands start to resemble that plane. In the middle of it all is
the city of Sigil, a city shaped like a ring, floating an infinite distance
above the Outlands.
The Astral Plane. A
silvery void that connects the outer planes with the prime material plane. Don't
let its nothingness fool you; this place is dangerous. The Githyanki make this
place their home. Another notable feature of the Astral Plane is that there are
corpses of gods here... Some are used by the Githyanki as floating fortresses.
The Ethereal Plane.
This grey fog connects the inner planes to the prime material plane. It's a bit
more peaceful than the Astral Plane, but it still has its dangers. Like demi-planes.
These were created by powerful wizards as "pet-planes". These wizards, as a
rule, don't like to be disturbed.
The city of Sigil merits some special attention. Not
just because it's shaped like a ring, but because it's seen as the center of the
Multiverse. It is 100% neutral ground, enforced by a being known as The Lady of
Pain, or just The Lady. This being has incredible power. She has the power to
keep the gods out of Sigil (which she does) or to imprison any individual that
is threatening to endanger the neutral character of Sigil. It has portals to
virtually every single plane.
