Duke began as a Roman word for an army commander without formal rank
The grand title of duke descends from the Latin dux, simply meaning leader. In republican Rome it described a military commander with no official rank; under Diocletian it became each province's top soldier. From there it passed to Germanic kings, Byzantine admirals and English princes, and today only Luxembourg is still ruled by one, a grand duke.
Diocletian's reforms split civil and military government in Roman provinces, and the dux took charge of the troops. In the eastern empire the word became doux, and in the 11th century the Byzantines created the Megas Doux as commander of the whole navy. In the West, after Roman power collapsed, the new Germanic kingdoms kept using the title for rulers of old provinces.
Each kingdom bent it differently. Visigothic dukes in Iberia were the greatest landowners and, alongside bishops, elected their kings. The Lombard dukes of Italy were so independent that after King Cleph died in 575 they ran the realm without any king for about a decade, before electing a new one and even shrinking their own lands to endow him. Among the Franks a duke was at first a temporary military chief over several counts rather than lord of a fixed territory; late Merovingian mayors of the palace styled themselves duke and prince of the Franks, a usage that lasted until 987.
England came late to the rank. Anglo-Saxon chronicles called the highest nobles below the king duces, but the local title was ealdorman, later earl. Edward III created the first English dukedom in 1337, making his eldest son, the Black Prince, Duke of Cornwall, and every one of his five surviving sons eventually became a duke. Two of those lines, York and Lancaster, would fight the Wars of the Roses. Sixteen ducal titles existed by 1483, yet after Bosworth in 1485 only four survived. Norfolk, forfeited when its duke died beside Richard III, was restored to the Howards thirty years later and remains the premier English dukedom.
The distinction between title and estate still matters in Britain. The Duke of Cornwall draws his income from a real duchy, while the Duke of Rothesay holds a title with no lands at all. The monarch owns the Duchy of Lancaster separately from the Crown, and islanders in Jersey and Guernsey toast the King as their Duke.
Source: Duke
