Succession system in traditional culture: what is the meaning of the first-born son

This is the system of succession of the first-born son in the patriarchal system of law, which means that the heir to the throne must be the first-born son of the king, no matter whether he is wise or not. That is to say, the heir to the throne must first be the king's first-born son, and among all the king's sons, the age of the youngest will determine who will inherit, that is, whoever is the eldest, who is the heir to the throne. The meaning of "to set up a son by his age, not by his age" is: although the heir to the throne is his own son, it is possible that he is not born of the same mother, and there may be a situation in which the eldest son is born out of wedlock (the oldest son is not born of his own queen), then "to set up a son by his age, not by his age" is used to determine the heir. "In other words, the heir to the throne must be the eldest son born to the queen. If the mother of the oldest son was a concubine, as long as the legitimate queen had a son, no matter how much younger he was than the son born to the concubine, the son born to the legitimate queen was to be established, and only if the legitimate queen did not have a son could the son of the concubine of the highest status be established as the heir, no matter what his age was. From this, it seems that if a person was the firstborn son and was born to the rightful queen, then he could definitely inherit the throne. That is why it is hard to be the firstborn son of the first son.