As I explained in the post on alignment, the cosmology elucidated in this blog draws on the original Moorcock model of law and chaos with the world in (and of) the balance. The world as we it know it is both in the balance, fought over by the forces of law and chaos, and of the balance, for it represents the harmony of law and chaos.
For the purposes of this dissertation, the planes are arranged in a model that the 3rd edition Manual of the Planes referred to as the "omniverse" and which formed the basis of the much maligned 4th edition. Fear not, there will be alterations to make it more friendly with with my fellow Planescape and Spelljammer partisans and I will not be lumping demons and devil together or elsewhere they don't belong. However, I will not be slavishly adhering to things like the patchwork terminology resulting from D&D's long line of writers.
The world with which we are most familiar is the Natural World, the Prime Material Plane, the world in (and of) the balance. It is the typical world for adventuring, full of many different planets and civilizations. However, it is merely one of many worlds floating within the Ethereal Sea, known collectively as the Preternatural Planes. Many of these worlds resemble the Material, and it is believed some of them may be reflections of it, that the Material is a reflection of them, or both. It here that the forces of balance reside, protecting and overseeing the natural world.
"Outside" the world are the Inner Planes or Inner Sphere, in loose accordance with the Aristotelian model of the planets. The Inner Sphere is another battle ground for the forces of law and chaos. The islands of stability are the work of law, with the greatest being the Elemental Planes or Elemental Poles. Each of the Elemental Poles represents a classical element like air, fire, earth and water. That is not to say that they are featureless expanses of that element, oh no. Each of the Elemental Poles has many varied landscapes just like the planets of the Material, with skies and lands and seas that reflect their element. However, between the poles exist the Elemental Chaos, where the elements mix and take forms that don't exist under the auspices of law and may be shaped by those with strong wills. Indeed, the Elemental Chaos predates the Elemental Poles. It is the Void from which the universe sprang, Ginnungagap, Tarassein and Limbo. It is home to the races of chaos: elementals, chaosiics, proteans, ogdoad and countless others.
"Above" the Inner Sphere (insofar as that direction makes sense) are the Upper Planes, ruled by the archons (a gnostic greek term for "ruler" and from which all "-arch" and "-archy" words originate) and their legions of celestials. The archons oversee the Celestial Bureaucracy that codifies and imposes the laws of physics that make a consistent existence possible (a concept burrowed from Chinese folk mythology). The upper planes are where you find angels, axiomites and the many other servants of law and goodness. That isn't to say that law is all good, as the more moderate celestials may disagree with the excesses of law.
"Below" are the Lower Planes. This is where the worst excesses of chaos have made their abode: qlippoth, bosch, demons and the souls of the damned. But it is also home to the worst excesses of law as well. During the conflicts between the gods and the demons, the demons devised new strategies on the fly to combat their enemies. The gods were forced to defy tradition and improvise as well: they sent the lawful devils to compete with the demons. It is unclear if the devils were created for this purpose, consigned to it to prevent them from turning against their moderate fellows, or fell from grace for perceived transgressions, but such distinction matters no longer. Ironically, this makes both the temptations of evil and all changes for the better into manifestations of chaos.