After the political fragmentation of the “Five Dynasties and Ten Kingdoms” period, the Northern Song consolidated much of the lands under these regional states into a larger polity in a process often described as “reunification.” But this “reunification,” judged against the domain of the Tang dynasty, was incomplete. The “Sixteen Prefectures” to the northeast were ceded to the Liao, and this became a vexing issue for Song emperors and officials. But the northeast was not the only region once under Tang rule that did not enter the Song domain. In this article, I discuss the area to the northwest of the Song, much of which was eventually governed by the Tangut Xia state. This area, roughly the modern provinces of Ningxia and Gansu, featured prominently in Northern Song political discussions, national geographical treatises, and national and regional maps. By analyzing the treatment of the northwest in these diverse genres of representation, I demonstrate a spectrum in the perceptions of the northwest. It was sometimes seen as little different from areas under Song rule; in other cases, it was treated as “beyond the sphere of civilization” (huawai). Such ambiguity is visualized in many Song cartographers who placed this area between two segments of the Great Wall. For Song emperors and officials, the northwest sat uncomfortably in their imaginations of the world, not easily dismissed and forgotten, yet irrecoverable.