"In retrospect, throughout the numerous changes in dynasties, when a new dynasty was established, they would not annihilate the previous dynasty's imperial lineage, but instead often treated them with considerable kindness and favor.
For example, when the Shang Dynasty overthrew the Xia, they merely banished Emperor Jie of Xia.
When the Zhou defeated the Shang, they even ennobled the royal family of the Shang Dynasty, second only to their own royal clan. After the Qin extinguished the six states, the aristocrats of those states remained.
It was only later, after the Sima Family claimed the throne and annihilated the Cao Family Descendants, breaking the rule, that this unwritten law was no longer observed by anyone. However, the Sima Family's end was not a good one either—they were exterminated to the last.
"Alas, the descendants have been unfilial, making the Star Lord a laughingstock."