When it comes to Chang 'an, we have to mention Luoyang, the two most famous capitals in history. Usually, the relationship between them is equal, and the unified dynasty with Chang 'an as its capital has never despised the management of Luoyang.
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Chang 'an is the most famous ancient capital in the history of China. It is "based on the solid foundation of Kanhan, defending the land of Yongzhou", with Tongguan in the east, Daguanguan in the west, Wuguan in the south and Xiaoguan in the north, which is quite easy to defend but difficult to attack.
Not only is it safe, but the Guanzhong area where Chang 'an once lived is also rich in fertile land, a thousand miles away. It is the earliest "land of abundance" and an ideal capital in all aspects. Therefore, from the pre-Qin dynasty to the Tang dynasty, it has always been valued by all previous dynasties.
Luoyang was not as large as Chang 'an for a long time. Luoyang in the Sui and Tang Dynasties was usually only half that of Chang 'an, and although Luoyang had many dangers, it was surrounded by great plains, so it was difficult to compare with Chang 'an militarily.
However, Luoyang's greatest advantage is that it is the center of the world with developed land and water transportation. In ancient times, there was a saying that Chang 'an, the capital, was conducive to dealing with rebellion, while Luoyang, the capital, was more suitable for educating the world with morality.