Yun Prefecture (Shanxi) in the context of Zhou (country subdivision)

The Sixteen Prefectures, more precisely known as the Sixteen Prefectures of Yan-Yun (traditional Chinese: 燕雲十六州; simplified Chinese: 燕云十六州; pinyin: Yānyún Shíliù Zhōu) or the Sixteen Prefectures of You-Ji (Chinese: 幽薊十六州; pinyin: Yōujì Shíliù Zhōu), were a historical region in North China that comprises present-day Beijing, Tianjin and parts of northern Hebei and Shanxi provinces. Situated around the Yan Mountains along the Great Wall, it was a strategic gateway region into the Central Plains and thus marred by constant military conflicts between various states for centuries since the end of the Tang dynasty until the establishment of the Yuan dynasty.