Transitive Planes are planes which exist among all the others, connecting them together. There are 3 commonly accepted Transitive planes and a fourth unnamed theoretical one.
Astral Plane: It is the only Transitive Plane to connect everything in the cosmos, including the Far Realm and the Dreaming.
Ethereal Plane: Connects the Prime Material, The Inner and Outer Planes.
Shadow Plane: Connects the Prime Material and the Outer Planes.
Theoretical Fourth Transitive: Connects the Prime Material to Other Prime Materials in Mirror Dimensions.