What is "guanzhong"

Guanzhong (Kwanchung; ), or Guanzhong Plain, is a historical region of China corresponding to the lower valley of the Wei River. It is called Guanzhong or 'within the passes', as opposed to 'Guandong' or 'east of the pass', i.e., the North China Plain. The North China Plain is bordered on the west by mountains. The Yellow River cuts through the mountains at the Hangu Pass separating Guanzhong from Guandong.