The so-called convenience of mountains and rivers means that when demarcation is made, it is divided by natural conditions such as mountains and rivers. For example, when Emperor Taizong was in power, the world was divided into ten roads, basically according to mountains and rivers, such as Guannei Road (west of Tongguan and north of Qinling), Henan Road (south of Yellow River and north of Huaihe River), Hebei Road (north of Yellow River and east of Taihang Mountain), Hedong Road (east of Yellow River and west of Taihang Mountain) and Longyou Road (west of Longshan). -as for the later increase to fifteen, I won't mention it for the time being.
This division method was very common before the Song Dynasty. For example, the Qin dynasty was divided into 36 counties, and the Han dynasty was divided into 13 States mostly because of the shape of mountains and rivers.
The benefits of this division are obvious, because natural mountains, rivers and lakes are the most direct and obvious differences between the two regions, and because of the barrier of mountains and rivers, there are often significant differences in culture, society and economy between the two regions. Using the principle of dividing mountains and rivers, administrative divisions and geographical areas are consistent and natural.
One of the most obvious examples of jagged teeth is that Hanzhong, which obviously belongs to Sichuan in terms of geographical unit and social culture, was included in Shaanxi Province. Gu Zuyu's Minutes of Reading History and Geography said that Hanzhong "is related to China in the north, Bashu in the south, Xiang and Deng in the east and Qin and Long in the west, and the situation is the most serious". Kong Zhang said, "The terrain in Hanzhong is dangerous and solid, and the four mountains are not as good as it." Huang Quan said, "If we lose Hanzhong, the No.3 bus will be depressed." Yang Hong said: "Hanzhong is the throat of Yizhou. Without Hanzhong, there would be no Shu State. " It is precisely because Hanzhong is so important to Sichuan that people who are independent of Sichuan have occupied Hanzhong since ancient times. However, the Yuan Dynasty set up a province, but Hanzhong was assigned to Shaanxi. Since then, Ming, Qing, Republic of China and even now have followed suit, Hanzhong entered Shaanxi, but did not dare to give it to Sichuan. Because once Hanzhong was transferred to Sichuan, Sichuan's victory was immediately complete, and it had the conditions for a separatist regime.