Thirty years in Hedong, thirty years in Hexi "was originally a folk proverb. The original explanation was that the Yellow River was diverted. Historically, the Yellow River has always changed its course, and it has basically changed in 30 years. This place was the west of the river 30 years ago, and it became the east of the river 30 years later.
In ancient times, due to the high riverbed of the Yellow River, the sediment deposition was serious, and the river course was not fixed and often flooded. Therefore, the Yellow River often changes its course. After changing the course, what was originally on the east side of the river is likely to be changed to the west side of the river. "Thirty years in Hedong, thirty years in Hexi", the so-called dubious advantage. East and west refer to the north and south banks of the river. Because the Yellow River often changes its course, it is used to describe the ups and downs of personnel. Nevertheless, during the Warring States period, Qin and Wei competed for the land area of Hanoi. Wu Qi helped Wei win Hanoi, and after 30 years, Bai Qi helped Qin recapture Hanoi. Folk songs are everywhere: 30 Hedong, 30 Hexi; Wu Bai set out twice, and the sky was a mystery. Thirty years in Hedong, thirty years in Hexi. "Generally speaking, personnel are changeable. Sitting in the village today may be someone else tomorrow. Today, there is a poem that says: East 30, West 40, which was ridiculous in those days. That's what it means. As can be seen from the figure of 60 Jiazi, the period from Jiazi to Wu Jia is relative, which happens to be 30 years. I think this should be the reason why people say "thirty years east of the river, thirty years west of the river".