Which Great Wheel. Because the 5e Great Wheel is not the Classic Great Wheel; the 5e Great Wheel is at its core the World Axis with a weird Astral Sea. If you look at the 5e Great Wheel you find that at the heart of it is the Feywild and the Shadowfell bordering on the Material Plane, and there's the Elemental Chaos surrounding that, and the Upper and Lower Planes surrounding the Elemental Chaos.
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If we look at the World Axis we find it's more organic and more of a mess, but everything is there. The Upper and Lower planes are all part of the Astral Sea - and if you were to force a structure on it you could put it in the shape of the Great Wheel. Just about the only thing you don't find in the World Axis that you do in the modern Great Wheel is the Ethereal Plane.
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The original Great Wheel was this accountant's design (you can tell really old versions because the LN plane is Nirvana not Mechanus; I think this illustration comes from the AD&D 1e DMG). It's symmetric and centered around cosmology. It looks as if there was an Overgod in control of it who would throw the Gods out of heaven if they didn't uphold this structure and uphold this artificial balance. (For those not aware this was the inciting incident of the Avatar Trilogy written to explain the changes between the Forgotten Realms of AD&D 1e and AD&D 2e). There was, of course, an intermediate set of planes (which is why the wheel is a square). The elemental planes and the planes like
the "Positive Material plane" (a.k.a. the Positive Energy Plane) are sometimes noted as very rarely having inhabitants.
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Where does it say that? As mentioned the Forgotten Realms cosmology at least used to be presented as objectively true and enforced. But then the Realms developed the World Tree... Honestly the World Axis reminds me of a map made by mortals for mortals in the old school style with the fantasy equivalent of "Here be Dragons" marked on it. And of
course it focuses on adventurable areas. I don't understand what peoples' problem is with a map made for mortals focusing on the parts that are useful to mortals.
Meanwhile the TSR Era Great Wheel, which
@Micah Sweet claims is mythology interpreted by mortal scholars is
far too defined for that. It slots everything into an equal 16 point wheel or the inner hub. Mythology is messy. The "Everything necessarily fits
this order" (as opposed to an Aarne-Thompson or Dewey Decimal approach of "We need
some ordering to find anything in this mess and this order is very much imperfect but it is at least an order") isn't the work of good
scholars or good
mythologists. It's the approach of tin foil hat wearing cranks in which there can be no accident or coincidence and in which there is nothing that doesn't fit the pre-ordained pattern that the crank brought with them to the show.