The administrative region of Cang Prefecture in the Tang dynasty was in modern eastern Hebei, Tianjin, and northwestern Shandong. It probably included parts of modern:
The imperial prefecture was the source of the modern name of Cangzhou, established in 1961.
References
Shi Weile, ed. (2005). Zhongguo Lishi Diming Da Cidian (中国历史地名大词典)[Large Dictionary of Chinese Historical Place Names] (in Chinese). China Social Sciences Press. p.1343. ISBN7-5004-4929-1.