Coats of arms of prince electors surround the Holy Roman Emperor's; from flags book of Jacob Köbel
(1545). Electors voted in an Imperial Diet for a new Holy Roman Emperor.
From the time of Constantine I (4th century), the Roman emperors had, with very few exceptions, taken on a role as promoters and defenders of Christianity.
The title of Emperor became defunct in Western Europe after the death of Julius Nepos in AD 480, although the rulers of the barbarian kingdoms continued to recognize the Eastern Emperor at least nominally well into the 6th century; both the title and connection between Emperor and Church continued in the Eastern Roman Empire until 1453, when it fell to the forces of the Ottoman Empire.
In the west, the title of Emperor (Imperator) was revived in 800, which also renewed ideas of imperial–papal cooperation. As the power of the papacy grew during the Middle Ages, popes and emperors came into conflict over church administration. The best-known and most bitter conflict was that known as the investiture controversy, fought during the 11th century between Henry IV and Pope Gregory VII.
After Charlemagne was crowned Emperor of the Romans (Imperator Romanorum) by Pope Leo III, his successors maintained the title until the death of Berengar I of Italy in 924. No pope appointed an emperor again until the coronation of Otto the Great in 962. Under Otto and his successors, much of the former Carolingian kingdom of Eastern Francia fell within the boundaries of the Holy Roman Empire. The various German princes elected one of their peers as King of the Germans, after which he would be crowned as emperor by the Pope. After Charles V's coronation, all succeeding emperors were called elected Emperor due to the lack of papal coronation, but for all practical purposes, they were simply called emperors.
The term sacrum (i.e., "holy") in connection with the German Roman Empire was first used in 1157 under Frederick I Barbarossa. Charles V was the last Holy Roman Emperor to be crowned by the Pope (1530). The final Holy Roman Emperor-elect, Francis II, abdicated in 1806 during the Napoleonic Wars that saw the Empire's final dissolution.
The standard designation of the Holy Roman Emperor was "August Emperor of the Romans" (Romanorum Imperator Augustus). When Charlemagne was crowned in 800, he was styled as "most serene Augustus, crowned by God, great and pacific emperor, governing the Roman Empire," thus constituting the elements of "Holy" and "Roman" in the imperial title.
The word Roman was a reflection of the principle of translatio imperii (or in this case restauratio imperii) that regarded the (Germanic) Holy Roman Emperors as the inheritors of the title of Emperor of the Western Roman Empire, despite the continued existence of the Eastern Roman Empire.
In German-language historiography, the term Römisch-deutscher Kaiser ("Roman-German emperor") is used to distinguish the title from that of Roman Emperor on one hand, and that of German Emperor (Deutscher Kaiser) on the other. The English term "Holy Roman Emperor" is a modern shorthand for "emperor of the Holy Roman Empire" not corresponding to the historical style or title, i.e., the adjective "holy" is not intended as modifying "emperor"; the English term "Holy Roman Emperor" gains currency in the interbellum period (1920s to 1930s); formerly the title had also been rendered "German-Roman emperor" in English.