The Byzantine Empire did not use heraldry as the Western Europeans did. For this reason, the Eastern Roman Empire did not have an official flag until after the marriage of Charlemagne and Irene. Even then, the Byzantines were not known for flying flags well into the 9th century. However, the Labarum symbol has been consistently synonymous with the Byzantine Empire, leading to the common misconception that it was, in fact, a flag of the Byzantine Empire.
Before the marriage of Irene and Charlemagne, the Byzantine Empire did not fly flags. The symbols carried by the late Western Roman Empire carried over to the Byzantine Empire, albeit with a few changes. Eagles, for example, were much less common in the Eastern Roman Empire than they had been in the Western or unified Roman Empire. Due to isolation, the necessity of carrying a flag seemed unimportant to pre-Unification Byzantines. Aside from familial crests, flags were not present. A common symbol, however, has been the Labarum. Identified by the Greek letters Chi (χ) and Rho (ρ), the Chi-Rho symbol is synonymous with the Byzantine Empire before and after the creation of the Carolingian Union and well into the Carolingian Empire.
Following the creation of the Carolingian Union by the marriage of Irene of Athens and Charlemagne, the Frankish Oriflamme flown by Charlemagne's men in his many wars was remodeled. Now flying a 'Byzantine shade of purple', this Oriflamme (Oriflamme du Irene) was created to show unity between the Franks and the Byzantines, who shared cold relations at the time of the conception of this union. Flown until 854, the Oriflamme du Irene was used by both the Frankish Empire and Byzantine Empires, albeit with their own changes. The Franks generally left the Oriflamme bare while the Byzantines were keen to place a Chi-Rho above a crossbar holding the Oriflamme.
In 849, the Carolingian Union was embroiled in the First Viking War. Led by Atticus I, the Franks and the Byzantines fought the Vikings out of the Rhine River. Following the bypassing of many traditions involving hereditary rule by Atticus in order to rule both the Franks and the Byzantines, the Oriflamme du Irene was redesigned. Angering the Franks, the Chi-Rho symbol was placed inside the depiction of a sun. The Chi-Rho was a blood red color to symbolize the blood shed during the First Viking Wars. The Oriflamme du Atticus lasted until Constantine VII's rule.
Constantine VII was concerned about Frankish dominance as much as he was afraid of Byzantine dominance. In a bold move, Constantine VII returned the Oriflamme to its red background as opposed to the royal purple background. Retaining the Chi-Rho, the Oriflamme was distinctly Byzantine and Frankish at the same time. The Oriflamme remained this way until Christophorus' decision to change the flag of the Carolingian Empire to a blue background, stating, "We are neither Frankish nor are we Byzantine now."