Outer Planes
If the Inner Planes are the raw matter and energy that makes up the multiverse, the Outer Planes are the direction, thought and purpose for such construction. Accordingly, many sages refer to the Outer Planes as divine planes, spiritual planes, or godly planes, for the Outer Planes are best known as the homes of deities.
When discussing anything to do with deities, the language used must be highly metaphorical. Their actual homes are not literally “places” at all, but exemplify the idea that the Outer Planes are realms of thought and spirit. As with the Elemental Planes, one can imagine the perceptible part of the Outer Planes as a sort of border region, while extensive spiritual regions lie beyond ordinary sensory experience.
Even in those perceptible regions, appearances can be deceptive. Initially, many of the Outer Planes appear hospitable and familiar to natives of the Material Plane. But the landscape can change at the whims of the powerful forces that live on the Outer Planes. The desires of the mighty forces that dwell on these planes can remake them completely, effectively erasing and rebuilding existence itself to better fulfill their own needs.
Distance is a virtually meaningless concept on the Outer Planes. The perceptible regions of the planes often seem quite small, but they can also stretch on to what seems like infinity. It might be possible to take a guided tour of the Nine Hells, from the first layer to the ninth, in a single day—if the powers of the Hells desire it. Or it could take weeks for travelers to make a grueling trek across a single layer.
The most well-known Outer Planes are a group of sixteen planes that correspond to the eight alignments (excluding neutrality) and the shades of distinction between them.
Demiplanes
Demiplanes are small extradimensional spaces with their own unique rules. They are pieces of reality that don’t seem to fit anywhere else. Demiplanes come into being by a variety of means. Some are created by spells, such as demiplane, or generated at the desire of a powerful deity or other force. They may exist naturally, as a fold of existing reality that has been pinched off from the rest of the multiverse, or as a baby universe growing in power. A given demiplane can be entered through a single point where it touches another plane. Theoretically, a plane shift spell can also carry travelers to a demiplane, but the proper frequency required for the tuning fork is extremely hard to acquire. The gate spell is more reliable, assuming the caster knows of the demiplane.
Outer Planes
The planes with some element of good in their nature are called the Upper Planes. Celestial creatures such as angels and pegasi dwell in the Upper Planes. Planes with some element of evil are the Lower Planes. Fiends such as demons and devils dwell in the Lower Planes. A plane’s alignment is its essence, and a character whose alignment doesn’t match the plane’s experiences a profound sense of dissonance there. When a good creature visits Elysium, for example (a neutral good Upper Plane), it feels in tune with the plane, but an evil creature feels out of tune and more than a little uncomfortable.