Neighborhood

The Neighborhood, also referred to as Neighbor World (の), is the world from another dimension where Neighbors and Trion Soldiers originated, including Yūma Kuga.

Each country travels across the sea of darkness on their own orbits, like average celestial bodies. Yūgo Kuga called this existence "Planet Nation". Their movements are a bit different from those of planets orbiting the sun, but many of planet nations brush by human world while revolving closer and farther. Every planet runs on a star (which itself is a Trigger). The star must be kept fueled, or it will die, and the land would eventually crumble. Only when they're close to Earth, that they can send out their Expedition Ship and open an interdimensional gate to carry out an invasion. Certain planet nations don't have fixed orbits and fly around freely in the Neighborhood. These have been called "Errant Nations".

Geography
There are currently 9 known nations:

Orbiting Nations

 * Liberi: The maritime nation surrounded by a rich, vast sea.
 * Leoforio: The cavalry nation where battles are fought on special Trion Soldiers.
 * Kion: The snow country where harsh terrain and climate thwart enemies from attacking.
 * Aftokrator: The god's country and the largest military state.
 * Galopoula: A vassal state of Aftokrator.
 * Rodochroun: A vassal state of Aftokrator.

Errant Nations

 * Ergates: A nation from which Xeno, Lilith, Gieve and Charon come.

Unknown

 * Calvaria: A defense nation which was at war with Spinthir.
 * Spinthir: A nation which was at war with Calvaria.
 * Menoeides: The nation which the Expedition Force, consisting of Tachikawa, Fuyushima and Kazama Units, departed from in the Black Trigger Retrieval Arc.

Trivia

 * 1) In the mini-skit at the end of the episode, in which the Neighborhood is described, the diagram of orbits resembles a classical diagram of the atomic nucleus.