Created in 1377, the title Duke derives from the Latin word Dux, meaning Leader, and is the highest form of non-royal nobility (although members of the Royal Family sometimes carry the title). In French, the term is Duc, and Italian Doge. Dukes in the United Kingdom are addressed as “Your Grace“. There are 27 Dukedoms in the peerages of England, Scotland, Great Britain, and the United Kingdom, held by 24 persons.
Duchy/Dukedom/Ducal – The Latin Dux was a military title that might roughly translate to “Field Marshal“. This historical kernel of in the stories of King Aurthor probably refers to a Dux bellorum in charge of the forces holding off the barbarian onslaught in the early post-Roman Britain.
The English King’s introduced the French Ducal structure into the British system, and it was initially a mostly Royal Title (as all new creations during the 20th century). In France especially after 1600, however, as well as in Britain, it has evolved into a mostly non-Royal title.
A Duchy (or Grand Duchy) is the territory ruled by a Duke (or Grand Duke) or the lands (and/or income) specifically attached to the Ducal title. A Dukedom is the title itself. In the UK, there are probably only two Duchies, those of Lancaster and Cornwall: these are essentially corporations holding properties that provide income for the Queen (who is Duke of Lancaster), and the Prince of Whales (who is also the Duke of Cornwall): as only these two Dukedoms carry such special “attachments” with the title. Duchies are this a royal preserve.
Duke is normally a very exalted title; however, when equating the dignity of some Dukes, some insight is needed. For example, Ferdinand of the Two Sicilies created Dukes in Naples almost by the gross, and these titles can’t be considered equal to Dukes in British or continental systems.