 Under the Qin, Yanmen was one of the commanderies which made up the principal divisions of the empire.Its seat was at Shanwu, south of present-day Youyu in Shanxi.Its territory ran from present-day Shanxi's Hequ, Wuzhai, and Ningwu Counties in the south to Inner Mongolia's Huangqi and Dai Lakes in the north; its eastern border was near Mount Heng.