Emperors from the same family are generally classified in historical periods known as Dynasties. Most of China's imperial rulers have commonly been considered members of the Han ethnicity, although recent scholarship tends to be careful about the dangers of applying current ethnic categories to historical situations. During the Yuan and Qing dynasties China was ruled by ethnic Mongolians and Manchurians respectively. A prominent historical view over the years sees these dynasties as non-native dynasties that were sinicized over time, while more recent writers argue that the interaction between politics and ethnicity was far more complex.
Chinese political theory does not totally discourage or prevent the rule of non-royals or foreigners under the title of the "Emperor of China". Historically, China has been divided, numerous times, into smaller kingdoms under separate rulers or warlords. The Emperor in most cases was the ruler of a united China, or must at least have claimed legitimate rule over all of China if he did not have de facto control. There have been a number of instances where there has been more than one "Emperor of All China" simultaneously in Chinese history. For example, various Ming Dynasty princes continued to claim the title after the founding of the Qing Dynasty (1644-1911), and Wu Sangui claimed the title during the Kangxi Emperor's reign. In dynasties founded by foreign conquering tribes that eventually became immersed in Chinese culture, politics, and society, the rulers would also take on the title of Emperor of China in addition to whatever titles they may have had from their original homeland. The most prominent example is Kublai Khan, who was both Great Khan of the Mongols and the Emperor of China.
The emperor's words were considered sacred edicts (聖旨), and his written proclamations "directives from above" (上諭). In theory, the emperor's orders were to be obeyed immediately. He was elevated above all commoners, nobility, and members of the imperial family. Addresses to the emperor were always to be formal and self-deprecatory, even by the closest of family members.
In practice, however, the power of the emperor varied between different emperors and different dynasties. Generally, in the Chinese dynastic cycle, Emperors founding a dynasty usually consolidated the empire through absolute rule, as evidenced in Emperors Shihuang of the Qin Dynasty, Taizong of the Tang Dynasty, Kublai Khan of the Yuan Dynasty, and Kangxi of the Qing Dynasty. These emperors ruled as absolute monarchs throughout their reign, maintaining a centralized grip on the country. During the Song Dynasty, the Emperor's power was significantly overshadowed by the power of the chancellor.
The Emperor's position, unless deposed in a rebellion, is always hereditary usually by agnatic primogeniture. As a result, there are many cases where a child Emperor ascends the throne when his father dies. When this occurs, the Empress Dowager, or the Emperor's mother, is in a position of significant power. In fact, the vast majority of female rulers during the entirety of Chinese Imperial history have come to power through ruling as regents on behalf of their sons; prominent examples include the Empress Lü of the Han Dynasty, as well as Empress Dowager Cixi and Empress Dowager Ci'an of the Qing Dynasty, who for a time ruled jointly as co-regents. If the Empress Dowager is too weak to assume power, court officials usually seize control. The presence of eunuchs in the court is also important in the power structure, as the Emperor usually relied on a few of them as confidants, which gave them access to many court documents. There are cases where eunuchs wielded absolute power, most prominent was the rule of eunuch Wei Zhongxian during the Ming Dynasty. There are also situations wherein other members of the nobility seized power as regents. The actual area ruled by the Emperor of China varied from dynasty to dynasty. In some cases, such as during the Southern Song dynasty, political power in East Asia was effectively split among several governments; nonetheless, the political fiction that there was but one ruler was maintained.
Unlike, for example, the Japanese monarchy, Chinese political theory allowed for a change in the ruling house. This was based on the concept of the Confucian "Mandate of Heaven". The theory behind this was that the Chinese emperor acted as the "Son of Heaven" and held a mandate to rule over everyone else in the world; but only as long as he served the people well. If the quality of rule became questionable due to repeated natural disasters such as flood or famine, or for other reasons, then rebellion was justified. This important concept legitimized the dynastic cycle or the change of dynasties.
This principle made it possible even for peasants to found a new dynasty, such as Han and Ming; and the establishment of conquest dynasties such as the Mongol-led Yuan Dynasty and Manchu-led Qing Dynasty. It was moral integrity and benevolent leadership that determined the holder of the "Mandate of Heaven." This Sinocentric concept, historians note, was one of the key reasons why imperial China in many ways had the most efficient system of government in ancient times.
There has been but one lawful reigning Empress in China, Empress Wu of the Tang dynasty or the Wu-Zhou dynasty founded by her. Many females, however, did become de facto leaders, usually as Empress Dowager. Prominent examples include Empress Dowager Cixi, mother of the Tongzhi Emperor (1861-1874), and aunt and adoptive mother of the Guangxu Emperor (1874-1908), who ruled China for 47 years (1861-1908), and the Empress Dowager Lü of the Han Dynasty.
As the emperor had, by law, an absolute position not to be challenged by anyone else, his subjects were to show the utmost respect in his presence, whether in direct conversation or otherwise. In a conversation with the emperor, it was considered a crime to compare oneself to the emperor in any way. It was taboo to refer to the emperor by his given name, even if it came from his own mother, who instead was to use Huangdi (Emperor), or simply Er ("son"). The emperor was never to be addressed as you. Anyone who spoke to the emperor was to address him as Bixia (陛下), corresponding to "Your Imperial Majesty", Huang Shang (皇上, lit. Emperor Above or Emperor Highness), tian zi (天子, lit. the Son of Heaven ), or Sheng Shang (聖上, lit. the Divine Above or the Holy Highness). Servants often addressed the emperor as Wan Sui Ye (萬歲爺, lit. Lord of Ten thousand years). The emperor referred to himself as Zhen (朕), translated into the royal "We", in front of his subjects, a practice reserved solely for the emperor.
Contrary to the Western convention of referring to a sovereign using a reign name (e.g. George V) or by a personal name (e.g. Queen Victoria), a governing emperor was to be referred to simply as Huangdi Bixia (皇帝陛下, His Majesty the Emperor) or Dangjin Huangshang (當今皇上, The Imperial Highness of the Present Time) when spoken about in the third person. He was usually styled His Imperial Majesty the Emperor of the Great [X] Dynasty, Son of Heaven, Lord of Ten Thousand Years. Forms of address varied considerably during the Yuan and Qing Dynasties.
An emperor also ruled with an era name (年號). Since the adoption of era name by Emperor Wu of Han and up until the Ming Dynasty, the sovereign conventionally changed the era name on a semi-regular basis during his reign. During the Ming and Qing Dynasties, emperors simply chose one era name for their entire reign, and people often referred to past emperors with that title. In earlier dynasties, the emperors were known with a temple name (廟號) given after their death. All emperors were also given a posthumous name (謚號), which was sometimes combined with the temple name (e.g. Emperor Shengzuren 聖祖仁皇帝 for Kangxi). The passing of an emperor was referred to as jiabeng (駕崩, lit. "collapse") and an emperor that had just died was referred to as Daxing Huangdi (大行皇帝).