Answer:
In the Tang Dynasty, in 618, Tang Gaozu made Chang 'an the capital, and in 657, Tang Gaozong established Luoyang as the capital, and since then, the western capital Chang 'an and the eastern capital Luoyang were listed as the capital, and the two capitals were equal, and in 684, the Empress Dowager Wu Zetian came to the court and called Luoyang the capital, "God capital", Chang 'an became the secondary capital. In 690, Wu Zetian changed the title of the state to Zhou, and in 692, Taiyuan, the northern capital, was added as the secondary capital, and the two capitals were restored in 705.
Chang 'an City of Tang Dynasty, Sui Dynasty called Da Xing City, it was built in Sui Dynasty, Tang Dynasty renamed Chang 'an City, the capital of Sui and Tang Dynasties, is the largest city in the world at that time. Luoyang City of Sui and Tang Dynasties, as the eastern capital of the Empire (Tokyo), together with Daxing City of Sui and Chang 'an City of Tang, was listed as the national political, economic and cultural center of the Sui and Tang Dynasties, and was one of the eastern starting points of the Silk Road. Chang 'an was the capital of the Tang Dynasty from 618-657, and Chang 'an from 657-684, Luoyang was the joint capital; 684-692 Capital Luoyang, secondary capital Chang 'an;
692-705 Capital Luoyang, secondary capital Chang 'an, Taiyuan; 705-723 Chang 'an and Luoyang as the capital; 723-757 Chang 'an Luoyang tied for capital, secondary capital Taiyuan; 757-762 Five capitals pattern, Chang 'an and Luoyang as the capital, Taiyuan, Fengxiang, Chengdu and other secondary capitals; 762-904 Capital Chang 'an, secondary capital Luoyang, Taiyuan; 904-907 The capital Luoyang.
