The characters Huang (皇 huáng "god-king") and Di (帝 dì "sage king") had been used separately and never consecutively (see Three August Ones and Five Emperors). The character was reserved for mythological rulers until the first emperor of Qin (Qin Shi Huang), who created a new title Huangdi (皇帝 in pinyin: huáng dì) for himself in 221 BCE, which is commonly translated as Emperor in English. This title continued in use until the fall of the Qing dynasty in 1911.
From the Han Dynasty, the title Huangdi could also be abbreviated to "huang" or "di." The former nobility titles Qing (卿), Daifu (大夫) and Shi (仕) became synonyms for court officials.
Although formally the Son of Heaven (天子, pinyin: tiānzǐ), where the Heaven refers to the celestial heavens or the universe, the power of the emperor varied between emperors and dynasties, with some emperors being absolute rulers and others being figureheads with actual power lying in the hands of court factions, eunuchs, the bureaucracy or noble families. In principle, the title of emperor was transmitted from father to son via primogeniture, as endorsed by Confucianism. However, there are many exceptions to this rule. For example, because the Emperor usually had many concubines, the first born of the queen (i.e. the wife) is usually the heir apparent. However, Emperors could elevate another more favoured child or the child of a favourite concubine to the status of Crown Prince. Disputes over succession occurred regularly and have led to a number of civil wars. In the Qing dynasty, primogeniture was abandoned altogether, with the designated heir kept secret until after the Emperor's death.
Unlike the Japanese emperor for example, Chinese political theory allowed for a change of dynasty as imperial families could be replaced. This is based on the concept of "Mandate of Heaven". The theory behind this was that the Chinese emperor acted as the "Son of Heaven". As the only legitimate ruler, his authority extended to "All under heaven" and had neighbors only in a geographical sense. He holds a mandate to which he had a valid claim to rule over (or to lead) everyone else in the world as long as he served the people well. If the ruler became immoral, then rebellion is justified and heaven would take away that mandate and give it to another. This single most important concept legitimized the dynastic cycle or the change of dynasties regardless of social or ethnic background. This principle made it possible for dynasties founded by non-noble families such as Han Dynasty and Ming Dynasty or non-ethnic Han 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." Every dynasty that self-consciously adopted this administrative practice powerfully reinforced this Sinocentric concept throughout the history of imperial China. Historians noted that this was one of the key reasons why imperial China in many ways had the most efficient system of government in ancient times.
Finally, it was generally not possible for a female to succeed to the throne and in the history of China there has only been one reigning Empress, Wu Zetian (624-705 CE) who usurped power under the Tang dynasty.
