There is a theory stating that around 5th century, they were called 'Kai Daio,' 'Kikoku Daio,' or simply 'Daio' (great king) externally, but it was fixed to 'Tenno' in the Reign of Empress Suiko or the Reign of Emperor Tenmu.
5世紀頃には対外的に「可畏天王」、「貴國天王」あるいは単に「天王」等と称していたものが推古朝または天武朝に「天皇」とされた等の説。