Duke (Lat. dux, a leader; Fr. due), a title belonging originally to the commanders of armies. In the later periods of the Roman empire it designated the military governor of a district, and until the time of Theodosius the rank of dukes was esteemed inferior to that of counts. Subsequently their dignity greatly increased, several provinces often became subject to a single duke, and the title was not disdained by conquerors such as Alaric and Attila. The northern barbarians who invaded the territories of declining Rome adopted, if they had not before borrowed, the titles of duke and count; but among these martial tribes the dukes, as military chieftains, acquired a decided preeminence over the counts, who both in the Byzantine and western empires had been employed chiefly in civil offices. Under the successors of Charlemagne, the governors of provinces generally assumed the title of duke, and achieved an almost absolute independence. The kings of France finally reunited to the crown the dukedoms which had been severed from it; and the ducal sovereignty being extinguished, the name has remained in France only as a hereditary title of dignity.

Prior to the revolution dukes were created by letters patent of the king, and were of three kinds, of which those designated as dukes and peers held the first rank, and had a seat in parliament, and certain honors and prerogatives at court. The dignity of the second class or hereditary dukes was transmissible to their male children, but that of the dukes by brevet ceased with themselves. The ducal and all other titles of nobility, abolished at the commencement of the revolution, were reestablished in 1806. The rank of duke in the royal family of France was superior to that of prince, inferior sometimes to that of count, and always to that of dauphin. In other great families also the title was higher than that of prince. In Germany, where the idea of sovereignty is inseparable from the ducal dignity, this title (Ger. Herzog) comes immediately after that of royalty. Under the emperor Henry IV. dukes began to usurp those sovereign rights which they have since exercised, and six dukedoms were then established. Several of the primitive dukes have exchanged their title for that of grand duke. The princes of the house of Austria bear the title of archduke.

The title of duke, or properly prince (Russ. Kniazh), was originally borne by the czars of Russia, and that of grand duke or grand prince still distinguishes the princes of that house. The kings of Poland were grand dukes or grand princes of Lithuania. Italy until recently had several ' sovereign dukes, as the grand duke of Tuscany, and the dukes of Modena and Parma. The title exists also in the Netherlands, and in Portugal and Spain. - In England the dukedom is the highest dignity in the peerage. It was introduced by Edward III., who in 1337 created his eldest son, Edward the Black Prince, then earl of Chester, duke of Cornwall, and subsequently prince of Wales, when the dukedom merged in the principality, and has ever since been vested in the heir apparent to the crown. The second dukedom was conferred, March 6, 1351, upon Henry Plantagenet, son and heir of the earl of Derby, under the title of duke of Lancaster, which dignity expired at his decease without male issue, but was reconferred in 1362 upon John of Gaunt, who had espoused the duke's second daughter, and eventually sole heiress, the Lady Blanche Plantagenet. In the reign of Elizabeth, in 1572, the whole order became extinct; but it was revived about 50 years afterward by her successor in the person of George Villiers, duke of Buckingham. A duke is styled "his grace" and the "most noble," and is officially addressed by the crown as "our right trusty and right entirely beloved cousin and councillor." The present prince of Wales, besides being duke of Cornwall, is duke of Rothsay; his brother Prince Alfred is duke of Edinburgh; the grandson of King George III. is duke of Cambridge; and besides those of the royal family there are in Great Britain 21 English, 8 Scottish, and 2 Irish dukes.