A year that begins with an annual number is called the first year. After the new emperor ascended the throne, he usually changed the year number of the calendar year, which is called "Gaiyuan". When the same emperor is in office, he can also change the year number. For example, Emperor Wu of the Han Dynasty changed the country name eleven times, using fourteen country names. In the early years of Tang Xuanzong's accession to the throne, it was renamed "congenital", which was called the first year of congenital, and later changed to "Kaiyuan" and "Tianbao". It was not until the Ming Dynasty that it was stipulated that one emperor and one yuan could be called the emperor by the year number. If the name of Ming Chengzu is Yongle, Ming Chengzu is called Emperor Yongle.
When the Japanese emperor ascended the throne, he would change his title. Before the Meiji Restoration, he always changed his title whenever something happened, so an emperor had at least three titles (even if he was in office for three years). During the Edo period, Emperor Meiji promulgated a law of one yuan for life after he ascended the throne.