China might broadly be divided into two regions based on its two major river systems: the Yellow River valley in the north and the Yangzi River valley in the south. The ancient political and cultural centers were concentrated in the north, but over the centuries the population and the economy shifted to the south. By 1250, the population of south China probably exceeded that of north China, and the largest (and richest) cities in the world were located in the south, starting with the city of Hangzhou with a population of more than one million.

Thanks to the monsoon winds that blow off the ocean from the southeast in the summertime, south China has enough water for intensive rice agriculture. The climate becomes drier and the rainfall more erratic as one moves north. North China is characterized by extensive cultivation of wheat, barley, millet, and sorghum. Farther north still, on the Mongolian steppe, there is insufficient rain to support agriculture, and the land is better suited for grazing livestock.

Since China was unified under the first Qin emperor in 221 b.c., periods of unity alternated with periods of disunity. The first period of unity lasted from 221 b.c. to a.d. 220, the same time that the Roman republic and empire ruled much of Europe. The second lasted from 589 to 756, the same time that Islam was spreading over the lands between Spain and Transoxania. The third lasted from 960 to 1127.