The term refers to the city in which a state ruler resided and conducted government affairs. The difference between a du (都) and a yi (邑) was that the former had an ancestral temple to enshrine the memorial tablets of ancestors and previous rulers while the latter did not. An ancestral temple used to be a place where rulers, the nobility, and senior officials made offerings to their ancestors. Therefore, an ancestral temple was a product of ancestral worshipping and a symbol of the patriarchal clan system. It is the defining structure of a du. During the Zhou Dynasty, the political center of all ducal states was called du. From the Qin and Han dynasties onward, du referred to the place where the emperor lived. Later, all cities large in scale and population were called du.
All cities with ancestral temples to house the memorial tablets of ancestors and previous rulers are called du while those without are called yi. (Zuo’s Commentary on The Spring and Autumn Annals)
When a city is called du, it is where the ruler of the land resides and where there is a large population. (Liu Xi: Explanation of Terms)