The Ring of the Heavens
After their victory over the primordials in ancient times, the gods decided to end the threat for good. In concert, they forged a great ring, a containment for the remaining and free-willed primordials, a containment for the Elemental Chaos.
On this ring, 16 "outer" planes were arranged in perfect symmetry, corresponding to the alignment of its inhabitants (lawful, chaotic, good, evil, and differentiations between). The Outands, a 17th, neutral plane capped the trapped Elemental Chaos and hid it from the sight of the gods.