Originally Posted by Kirialax
Note also that the emperor we call Constans II (r. 641-668) had the official regnal name of Constantine, thus adding to the confusion.
The regnal numbers of Roman, "Byzantine" and Holy Roman Emperors are sometimes all messed up.
Marcus Julius Philippus Augustus (or Philip the Arab) was emperor from 244-249 AD. His young son Marcus Julius Phlippus Severus (or Philip the Younger) was his co emperor.
According to the modern convention of listing rulers by their personal names followed by a Latin numeral they should be listed as Marcus XIV and Marcus XV if I remember correctly. But instead they are sometimes listed as Philip I and Philip II.
Philippikos Bardanes was eastern Roman or "Byzantine" emperor from 711 to 713. Since he as the first with the personal name of Philip he could be listed as Philip I. If he is listed as a successor of Philip the Arab and/or Philip the Younger he could be listed as Philip II or Philip III.
The Holy Roman Emperors after Otto I the Great claimed to be the successors of the Carolingian Emperors in succession to Charlemagne. When Charlemagne was crowned emperor in 800 he claimed to be the rightful successor of Constantine VI who had been deposed by his mother Irene in 797.
So the Holy Roman Emperors claimed to be the rightful successors of the Roman emperors and the eastern Roman or "Byzantine" emperors from 395 to 797 AD.
Philip, King of the Romans (romanorum rex et semper Augustus) from 1198 to 1208 could thus be listed as Philip I, Philip II. Philip III, or Philip IV, and is usually listed as Philip I when given a regnal number.
But he has been described as Philip II in a document from his own time:
Philippus secundus divina favente clementia Romanorum rex et semper augustus Empire and Germany. Titles of European hereditary rulers
And I wonder which previous Philip as counted to give him that number.