Sixteen Prefectures
The Sixteen Prefectures were administrative units established during the Tang dynasty. Under the Tang, each prefecture or "zhou" was a unit of administration larger than a county but smaller than a province. The Sixteen Prefectures stretched from Ji County in modern-day Tianjin Municipality to Datong in Shanxi Province, extending contiguously along the mountains that divide the agrarian plains of central China from the pastoralist steppes to the north. Several dynasties including the Qin and the Northern Dynasties before the Tang built the Great Wall along these mountains. Seven of the Sixteen Prefectures were located inside (south) of the Inner Great Wall. The other eleven were located in between the Inner and Outer Great Walls. The Tang did not build Great Walls but used frontier military commanders to guard against the northern tribes. The Fanyang or Youzhou-Jizhou Commandery, based in modern-day Beijing commanded 11 of the Sixteen Prefectures. The other seven were commanded by the Hedong Commandery based in Yunzhou, modern Datong.