When and where were the elemental planes first introduced in print? A magazine article likely?
3LBBs mention elementals and higher planes but not elemental planes.
I can't find any reference in Supps I, II, III, nor IV.
I know that the AD&D DMG mentions the elemental planes. So we're talking between those.
The Article Planes is in Dragon 8, July 1977. It mentions elemental planes in the first paragraph... (from the Best of Dragon Vol 1 reprint of it...)
Gary Gygax said:
For game purposes the DM is to assume the existence of an infinite number of co-existing planes. The normal plane for human-type life forms is the Prime Material Plane. A number of planes actually touch this one and are reached with relative ease. These planes are the Negative and Positive Material Planes, the Elemental Planes (air, earth, fire, water), the Etheriat Plane (which co-exists in exactly the same space as the Prime Material Plane), and the Astral Plane (which warps the dimension we know as length (distance)). Typical higher planes are the Seven Heavens, the Twin Paradises, and Elysium. The plane of ultimate Law is Nirvana, while the plane of ultimate Chaos (entropy) is Limbo. Typical lower planes are the Nine Hells, Hades' three glooms, and the 666 layers of the Abyss.
It's also got the beginings of the AD&D Great Wheel as an illustration.