Louis the Pious was co-Emperor along with his father starting in 813.
 Only the emperors who were recognized as legitimate rulers and exercised sovereign authority are included, to the exclusion of junior co-emperors (symbasileis) who never attained the status of sole or senior ruler, as well as of the various usurpers or rebels who claimed the imperial title
The Byzantine Empire was the direct legal continuation of the eastern half of the Roman Empire following the division of the Roman Empire in 395

