Sky (capitalized) is a term used in the Metal Shadow universe to refer to networks of deepes. The term originated because deepes hover high in the air, as well as create their own skylight, so hence, most of human civilization live their entire lives in the "sky".
Forms of transportation have been developed over the centuries to allow travel between most deepes in a Sky. All nations in Gilta Nnea associate themselves with their own network of deepes. The separation of these networks is arbitrary, but they have been numbered from one to twelve by the main group, with the order going from south to north. People who hail from most nations call themselves "people of the (according number) Sky".