Duke
The term duke is a title of nobility which refer to the sovereign male ruler of a Continental European duchy, to a nobleman of the highest grade of the British peerage, or to the highest rank of nobility in various other European countries, including Spain and France (in Italy, principe is held to be the highest grade). The wife of a duke, or a woman who rules a duchy, is known as a duchess.
The word duke derives from the Middle English, which in turn came via the Old French duc from the Latin duc/dux, which in turn came from the verb ducere, meaning "to lead".
In the United Kingdom, the office of a duke along with its dignities, privileges, and rights is a dukedom. However, the title of duke has never been associated with actual rule in this country. Dukes are addressed as 'Your Grace' and referred to with the prefix 'His Grace'.
Royal dukes
A royal duke is a duke who is a member of the royal family, entitled to the style of "His Royal Highness".There is nothing about the particular dukedom that makes it royal. Rather, it is called a royal dukedom because it is held by a member of the royal family.
In the United Kingdom, the current royal dukes are HRH the Prince of Wales, who is Duke of Cornwall and Rothesay; HRH the Duke of Edinburgh (Prince Philip), HRH the Duke of York (Prince Andrew), HRH the Duke of Gloucester (Prince Richard), and HRH the Duke of Kent (Prince Edward). Edward VIII became Duke of Windsor after his abdication. With the exceptions of the dukedoms of Cornwall and Rothesay (which can only be held by the eldest son of the reigning monarch), these other dukedoms are hereditary in the male line. Other royal dukedoms that have been awarded in the past include those of Albany, Avondale, Cambridge, Clarence, Connaught, Kendal, Stathearn, St. Andrews, and Sussex.
The heir apparent to the Belgian throne is titled Duke of Brabant.
Nowadays, Spanish infantes and infantas are usually given a dukedom upon marriage. This title is not hereditary. The current royal duchesses are: HRH the Duchess of Badajoz(Infanta Maria del Pilar), HRH the Duchess of Soria(Infanta Margarita), HRH the Duchess of Lugo(Infanta Elena) and HRH the Duchess of Palma de Mallorca(Infanta Cristina).
All Swedish princes and, since 1980, princesses are given a dukedom for life. Currently, there's one duke and two duchesses: HRH the Duchess of Västergötland(Crown Princess Victoria), HRH the Duke of Värmland(Prince Carl Philip) and HRH the Duchess of Hälsingland and Gästrikland(Princess Madeleine).
History
The Germanic Franks converted under Roman influence the Germanic concept of Herzog (literally: "war-leader"), the temporarily elected general for a major expedition of warfare, into military governors for units of up to a dozen counties. In the 7th century these units developed into hereditary clan-duchies of Bavarians, Thuringians, Alemanns, Franks and other Germanic tribes, which Charlemagne crushed in 788, converting the border provinces into margraviates (which however soon emerged as clan-margraviates: Saxony, Bavaria, Swabia, Lorraine...).
The dissolution tendency was counteracted by the appointment of younger sons of the monarchs (royal dukes) as military governors of the important border provinces, which however also soon developed into hereditary duchies and a source of intrigues against the monarch (see for instance: History of Schleswig-Holstein). The medieval dukes had a strong position in the realms they belonged to. Like the margraves, they were responsible for the military defence of an important region, and had strong arguments for retaining the Crown's tax incomes of their duchy to found their military force.
Although there are no longer any sovereign duchies (Luxembourg is a grand duchy), since the 19th century, there have been sovereign dukes of Parma and Modena in Italy, and of Brunswick, Anhalt, Saxe-Coburg-Gotha, Saxe-Meiningen, and Saxe-Altenburg in Germany.