The star systems of the Third Horizon are linked by portals constructed by ancient beings called, simply enough, the Portal Builders. Each system has at least two portals, one that allows entry into the system, and one that allows exit to at least one other system. A ship's drive is tuned to choose which system is their destination if there are multiple options. By these portals, the worlds of the Third Horizon are linked, allowing trade and a common culture. And it was by the destruction of portals that the paths to the First and Second Horizon were severed.