Why is the song dynasty divided into two periods, the northern song and the southern song?

The answer is

The Song dynasty was founded by Emperor Taizu of Song and is divided into two periods, the Northern Song and the Southern Song. The Northern Song's capital was in the northern city of Bianjing. The government was the first in world history to issue paper money. It controlled most of what is now Eastern China. Southern Song refers to the period in time when the Songs lost control of its northern half to the Jurchen Jin dynasty in the Jin-Song war and withdrew from south of the Yangtze and established a capital in what is now Hangzhou.

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