The so-called Di Zi refers to the son born to his wife, while the illegitimate child refers to the son born to a concubine or a concubine outside the house (that is, a concubine who has not officially entered the door and has no birthright). The so-called eldest son refers to the first son of the man, regardless of the identity of the biological mother. If the eldest son is born to a concubine or concubine, he is called the eldest son. The first son born to a wife, no matter how many lines, is called the eldest son. In other words, the so-called eldest son, eldest son, eldest son, only for the son born to his wife, is the ranking of the eldest son, regardless of the ranking among all sons.
For example, Liu Bang, the eldest son of Emperor Gaozu, mourned Liu Fei, the king of Qi, because his mother Cao Zhi was a consort of Emperor Gaozu when she was a child. Therefore, although he is older than Emperor Liu Ying, he is only the eldest son. Although Emperor Hui of Han Dynasty took the second place, he was the eldest son because he was born by Emperor Gaozu of Han Dynasty. For another example, Tang Gaozong Li Zhi was the ninth son of Emperor Taizong and Li Shimin. Because he is the third son of Emperor Taizong's original wife and the third son of Emperor Taizong, his patriarchal status is far above many other concubines.
There is a great difference between ancient Di Zi and illegitimate children. In the eyes of ordinary people, after the death of the old man, only the emperor is qualified to participate in the separation, while the illegitimate child is not qualified. At best, this bastard can only give certain property and set up another door when the old man is alive. Or the son who inherits the identity of the old man will give some property and set up another door. There are also some differences between sons who participate in the separation, and the eldest son is the first priority. Usually, ancestral houses and ancestral properties should be placed under the name of the eldest son. If it is in the family of nobility and emperor, it also involves the inheritance of title and throne. In dynasties that strictly follow the "eldest son inheritance system", usually only the eldest son inherits the title or the throne, and even the second son is not qualified. Of course, the Qing Dynasty and other dynasties did not strictly follow the "eldest son inheritance system".
However, the succession of the title and the throne is usually given priority to the younger son. For example, Wang Yixian, the thirteenth brother of Sejong in the Qing Dynasty, Xiao Hong, the heir of Wang Yixian, and Jiao Hong, the seal of Ningjun King, were all born to Zhao Jia, the eldest son of Wang Yixian, and they still take priority. But in fact, Xiao Hong's tooth order is after Jiao Hong's, that is to say, the inheritance of the title of Prince Yi did not strictly follow the "eldest son inheritance system", but was inherited by Xiao Hong with lower tooth order. However, in any case, the heirs are sons, not illegitimate children.
In addition to the difference between official and ordinary, there is actually the difference between official and subordinate. This involves the first wife, the second wife and the peaceful wife. The so-called original match is also the original match. The son of the first wife is the most authentic first son, and no one can challenge his position. Therefore, China strictly followed the "eldest son inheritance system" in ancient times. As long as the original queen has sons, usually the oldest of them will be awarded to the crown prince, except in special circumstances.