Three metric systems, also known as "three publics" and "three subjects". "Sanpin" is an important central administrative system in China feudal society. It is a central administrative system with a number of senior officials under the monarch who are in charge of administration, military affairs and supervision respectively, so that they can contain each other and form a central power center to serve centralized politics.

Sangong is the collective name of the three most prominent official positions in ancient times. The establishment of the Qin dynasty was mostly in vain after the Qin dynasty. It is said that this word was already used in the Zhou Dynasty. According to The Book of Rites and Biography, the scholars in the Western Han Dynasty thought that "the three public means Sima, Situ," and so on. According to Zhou Li, classical scholars in ancient China believed that a surname, a teacher and a Taibao were three kinds of public officials.

Different dynasties have different interpretations and titles of official positions, but they are all the three most prominent official positions in ancient times: in the Western Zhou Dynasty, the highest official positions similar to the three official positions were Taishi, Taifu and Taibao; The three fairs in the Western Han Dynasty were Da Situ, Fu and Da Sikong. During the Eastern Han Dynasty, it refers to Qiu, Situ,; In Qin dynasty, it refers to the three fairs in the three fairs and nine expensive systems, namely, the prime minister, Qiu, and the doctor too much. During the Ming and Qing Dynasties, Sangong became a Taishi, Taifu and Taibao. In addition, in the absence of specific mention, the three fairs are all about the three fairs and nine fairs in the system founded by Li Si in Qin Dynasty.