The title Duke (and Prince) of the Franks (Latin: dux (et princeps) Francorum) has been used for three different offices, always with "duke" implying military command and "prince" implying something approaching sovereign or regalian rights. The term "Franks" may refer to an ethnic group or to the inhabitants of a territory called Francia.
The first office was that of the mayors of the palace of the Merovingian kings of the Franks, whose powers increased as those of the kings declined. The second was that of the second-in-command to the early kings of France, the last incumbent of which succeeded to the throne in 987. This title was sometimes rendered as Duke of France (). The third instance was that of the rulers in East Francia (now Germany) of the so-called "tribal" duchy of Franconia.
Dux et princeps Francorum
Up until the time after Dagobert I, the title princeps (prince) had royal connotations. The first time it was used to describe the mayors of the palace of Neustria was in mid-7th-century saints' lives. The Vita Eligii refers to unspecified principes of the palatium of Neustria, and the Vita Baldechildis and Passio Leudegarii describe the mayors Erchinoald and Ebroin as princes. Pippin II first used the title princeps after his victory at the Battle of Tertry in 687. Both the Liber historiae Francorum and the Vita Dagoberti tertii refer to him by this title, but the continuation of the Chronicle of Fredegar uses only the title "duke". The historian Bede refers to Pippin II as dux Francorum, but the ninth-century Anglo-Saxon translator of Bede uses the term Froncna cyning (king of the Franks). The continuator of Fredegar refers to Ragamfred as a prince, but he only calls his rival, Pippin's son Charles Martel, a prince after his victory over Ragamfred in 718. The princely title was used continuously from this point on for Charles and his descendants, the Carolingians, both in narrative and charter sources.
In 742, Pippin's grandson, Carloman, held his great Concilium Germanicum, at which he addressed the assembled "servants of God and my great men ... who are in my kingdom" as "duke and prince of the Franks". The council declared that "without the patronage of the prince of the Franks it is impossible to defend the people of the church, the presbyters, clerics, monks and nuns of God." When in 744 Carloman's brother, Pippin III, used the same title, he did not refer to "my kingdom", since by then a Merovingian king, Childeric III, had been appointed. The ducal/princely title used by the early Carolingians marked them off as peers of the duke–princes of Aquitaine, also nominally under the Merovingians, rather than as sovereigns over them.
Dux Franciae
A charter from the reign of King Odo (888–898) for the church Saint-Aignan d'Orléans that titles Robert the Strong dux Francorum is a 17th-century forgery.
Between 936 and 943, the title dux Francorum was revived at the request of Hugh the Great, the most powerful magnate in France. A charter of King Louis IV of 936 refers to him that way, and a charter of Hugh's own from 937 employs the title. Its usage was not exclusive, however, since the title "count" (comes) continued to be used as well. In one charter, Louis explained that Hugh was second to him in all his kingdoms. This interpretation of the title dux Francorum was not universally accepted. According to Flodoard of Reims, the king only "invested Hugh with the duchy of France" in 943 in reward for the latter's help in returning the king from exile. The duchy of Francia (ducatus Franciae) comprised the region between the Loire and the Seine, the ancient kingdom of Neustria. On this contemporary understanding, Hugh's title was analogous to that of the dukes of Aquitaine, dux Aquitanorum, where the territorial designation ("of Aquitaine") was eschewed in favour of an ethnic one ("of the Aquitainians"). "Duke of the Franks" thus became the preferred title of the Robertian margraves of Neustria from 943. Walther Kienast suggested that the title served to connect Hugh with the previous duces Francorum, Pippin II and Charles Martel, and buttress his authority in Francia, from which he was frequently absent on visits to the royal court.