Ancient Chinese dynasties often set up a "capital" in addition to the official capital in the capital setting. The escort capital system has a long history, and in the Western Zhou Dynasty, there was already a companion capital Luoyi. Subsequent dynasties such as the Two Han Dynasties and the Tang and Song Dynasties basically followed the old system of accompanying the capital, and even the "two capitals and three capitals system" appeared in the Ming Dynasty.
In the division of the capital, the ancient emperors often distinguished by geographical orientation, so there were successively Beijing, Nanjing, Tokyo, Xijing, and even Shangjing and Zhongjing. For example, the Bohai State of the Tang Dynasty modeled on the Tang Dynasty system and established the "Five Capitals System".
And because the capitals of each dynasty are often different, Beijing, Nanjing, Tokyo, and Xijing, which are defined simply in terms of orientation, may refer to different cities or regions in different historical periods.
Beijing
The title of Beijing can be traced back to the Eastern Jin Dynasty, when Beijing referred to a small city, Tongwancheng. Judging from today's administrative divisions, Tongwancheng is roughly located in Jingbian County, Shaanxi. However, at that time, Tongwancheng was the capital of the Great Xia Kingdom. After Emperor Xiaowen of northern Wei moved the capital to Luoyang, Luoyang became the eastern capital, and the original capital, Taiyuan, became Beijing.
During the Tang Dynasty, the Li regime took Taiyuan, the land of Longxing, as Beijing. During the Five Dynasties period, most of the states also used Taiyuan as Beijing. During the Northern Song Dynasty, Daming Prefecture (present-day Daming County, Hebei) was established as Beijing and became the headquarters of the front line to block the Jin army from going south. After the Later Jin Dynasty moved to the new capital, it was also set up in Beijing, which roughly ranged from today's Ningcheng County in Inner Mongolia.
In the Ming Dynasty, after Ming Chengzu moved the capital to Suncheonfu, he changed The Shuntianfu to Beijing. Since then, the name of Beijing has not changed. Therefore, for a long time in history, Beijing refers to the current beijing city.
nanking
During the Tang Xuanzong Dynasty, the famous "Anshi Rebellion" broke out, and Xuanzong once went to Chengdu to escape the chaos of war. After Emperor Xuanzong came to power again, he built Chengdu into Nanjing.
Subsequently, the Khitan state also once took Dongping County as Nanjing. After the Liao state invaded and occupied Youyun Sixteen Prefectures, it used Youdu Prefecture (present-day Beijing City) as Nanjing. During the Northern Song Dynasty, Song Taizu took Song Prefecture (宋州, in modern Shangqiu County, Henan) as Nanjing.
In the Ming Dynasty, Ming Taizu Zhu Yuanzhang renamed Ying Tianfu as Nanjing, and set the capital here, from which Nanjing got its name. Since then, Nanjing has generally referred to the city of Nanjing, Jiangsu.
Tokyo
From the Eastern Han Dynasty to the Tang Dynasty, except for some changes, successive dynasties generally called Luoyang Tokyo. During the Five Dynasties period, the Later Tang Dynasty once made Xingtang Province (present-day Daming County, Hebei) the capital. Later Jin dynasty changed Kaifeng Province to Tokyo.
The Liao state also once renamed Nanjing (present-day Liaoyang, Liaoning) tokyo. However, in Chinese history, Tokyo generally refers to Luoyang.
Xijing
Both the Eastern Han Dynasty and the Sui Dynasty moved their capital to Luoyang, and in order to distinguish them from the eastern capital Luoyang, they renamed Xi'an as Xijing. During the Xianqing and Tianbao years of the Tang Dynasty, Xi'an was also called Xi'an the Western Capital, and Luoyang was the Eastern Capital. During the reign of Emperor Suzong of Tang, because Emperor Suzong had taken refuge in Fengxiang Province (present-day Fengxiang County, Shaanxi), after the war subsided, he once called Fengxiang Province Xijing.
During the Five Dynasties period, the Later Tang Dynasty took Taiyuan Prefecture (太原府, in present-day Taiyuan, Shanxi) as the Western Capital, and after the Jin Dynasty moved the capital to Kaifeng Prefecture, luoyang was the Western Capital. The Northern Song Dynasty also always referred to Luoyang as Xijing. During the Liao Dynasty, Datong Province (大同府) (present-day Datong City, Shanxi) was the western capital. During the Ming Chongzhen Dynasty, Zhang Xianzhong was proclaimed emperor in Chengdu, so Chengdu was also known as Xijing for a while.
By the end of the Ming Dynasty, Li Zicheng, the king of Chuang, had captured Xi'an and changed Xi'an to Xijing. After going around, the name of Xijing finally fell on Xi'an's head. Since then, Xijing has also generally referred to Xi'an.
In summary, for the vast majority of time in Chinese history, Beijing refers to Beijing City, Nanjing refers to Jiangsu Nanjing City, Tokyo refers to Luoyang, Henan, and Xijing refers to Xi'an, Shaanxi.