I’ve posted a few times on the planes of my campaign as I firmed up exactly what cosmological model I wanted (seems to be something going around; while this was sitting in my drafts, enworld had this and rpg.net had this).
I’ve been thinking about the distinction between a layer and a plane. “Layer” is just a way of breaking up the monotony of a generic location into actual adventurable locales; the Abyss is the plane of all the demons, but the three layers of Azzagrat are specific environs (under the control of Graz’zt). Other than the Abyss, D&D planes are classically made of a fixed number of physically concrete layers; the Seven Heavens contain seven layers, one per heaven-mountain. Cool and all; not what I’m into.
Here’s what I’m into:
Let’s start with the base concept of a Domain. Domains are locations which exist; they have borders. There’s Cathule (where my game is set), but also Barovia; Dis (my first layer of Hell) can be sailed to from Cathule but literally is in Hell, Graz’zt’s layer of Azzagrat, and so forth. The borders of a domain set the limits for things like teleportation and scrying; all points in Barovia are in Cathule and so can be scried, but not the other way around.
Domains organize into Realms. Realms can be defined by anything — Hell is a Realm, but the country of Dis and each of the worlds of Baator and each of the worlds of Naraka and of Purgatory and so forth, those are Realms. Sometimes the borders between two domains in the same realm are very soft, such that you can walk or sail between them (such as Cathule to Barovia, though not the other way around) and sometimes very hard (such as the various realms of the Abyss). Still, some form of transport is generally achievable, through the use of gates at a minimum.
Okay, so: No structure, I get to draw a line diagram. As long as the plane shift spell exists, does any of this matter? Yes-ish. In the same way I draw a campaign map even though we’re just going to skip over most travel montages, I should do the same thing for planar campaigns. In the same way the characters will pick between a coastal cog and the inland caravan, I should provide the Charonic Nether Skiff versus the Limbic Stormwalk. That’s where the “planes” of the Nether and Limbo start to matter: they’re really big Realms with not much content that exist to fill maps: they’re basically oceans, to the Realms’ islands, including such weird effects as “the sun rises in the east” under the similar-behavior “the dead wind up in the Nether”. Most of the time, you don’t have an adventure on the open ocean, you have it near a feature: in a realm.
Neat. But humans don’t work that way; they organize orreries and seating charts. So probably the Realms get called planes and hooked up in a big wheel, nice and far away from the infinite expanse of empty air called the Inner Plane of Air. But don’t be fooled: that’s the equivalent of a blank spot on a map or a barren rock covered with gull poop until some adventure comes along and needs it, making it concrete.