- Water holds everything liquid. Air holds everything that is a gas. Earth holds everything solid.
- Fire holds energy — the loose thermal energy that shows up in the physical planes. Fire itself is not an element; it is a reaction driven by heat. The plane is named for what people see.
- Light, Shadow, Life and Death hold every kind of their own. Death's purview includes undeath, the force that creates and powers magically made undead.
Each plane holds everything within its purview, in its purest form, and very little outside it. Matter inside them is layered by density, with each layer resting on the next. The seams between layers are inhabited.
Only what exists in nature. A substance is in its plane if it occurs naturally — even if it needs refining to be usable. Things that do not exist in nature at all, and must be made, are not. Find a natural occurrence of something and it belongs in its plane.
The elemental planes are incredibly hostile to life. They are sources, not destinations.
They are load-bearing. Each reality uses its elemental planes to pull and filter elemental forces and check how they work. If an elemental plane were cut off from its reality, the consequences for that reality would be dire.
Primordial elements have unique properties and take priority when put into a mix.