5E Great Wheel Model: Inner Planes
The four elemental planes—one each for Air, Fire, Earth, and Water—plus the two energy planes—one for positive energy and one for negative energy—made up the six major Inner planes.
Between each pair of adjacent elemental planes was a para-elemental plane. The eight quasi-elemental planes were found between the four elemental planes and the two energy planes. The four para- and eight quasi-elemental planes added up to twelve minor planes for a total of eighteen Inner planes.
Each plane was made up of primarily one type of matter, but pieces of other Inner planes may have found their way in (or been brought in by powerful forces). One thing all Inner planes had in common was that they were all hostile to lifeforms from outside, particularly from the Prime Material Planes.