Such background brought about the custom under which an emperor would pass the throne on to the successor at an early stage, and became a retired emperor to act as the top decision maker and dealt with foreign policy (China), while the new emperor dealt with domestic affairs.
そこで皇帝が早い段階で後継者に帝位を譲って太上皇となり王室内の最高意思決定と対外（中国）交渉を行い、皇帝は内政一般を扱うという慣習が成立した。