I was reading through Necromancer's City of Brass, specifically the chapter on the Plane of Molten Skies, and the geography started bugging me. How could this limited plane exist next to three infinite planes, and still leave room (somewhere) for a plane of water? I started drawing, and what I came up with is called a truncated tetrahedron on wikipedia - or a d4 with the points filed down flat. Each filed-down point of the d4 is a limited plane like the Plane of molten skies, with walls that stretch straight outward, at a 90 degree angle, from the filed-down point into infinity. The center of the d4 could be a confluence of all four elemental planes. The 4 elemental planes stretch outward from the 4 sides of the d4, getting larger as they get farther away, also out to infinity.
So it can all make sense in 3-dimensional space. Planes of positive and negative energy are obviously not included.
Since I had myself a little "wow" moment, thought I'd share, for whoever else gets bugged by these kind of things.
I'm developing a similar elemental plane arrange for my new campaign world. At the corners of the d4 (where 3 planes converge) I have the greater paraelemental planes (like Ice-Air/Water/Earth) and the edges (where 2 planes converge) are the lesser paraelemental planes (like Mud-Water and Earth) This has lead to all sorts of other weird ideas like elemental and paraelemental specialist mages and different supernatural powers/godlings like the Stormlord (Water and Air)