Duchy

A duchy is a country, territory, fief, or domain ruled by a duke or duchess. The term is used almost exclusively in Europe, where in the present day there is no sovereign duchy (i.e. with the status of a nation state) left.

The term "duke" (like the corresponding "duchy") should not be confused with the title Grand Duke (or Grand Duchy, such as the present-day Grand Duchy of Luxembourg), as there exists a significant difference of rank between the two.

In common European cultural heritage, a grand duke is the third highest monarchic rank, after emperor and king. Its synonym in many eastern European languages (Russian, Lithuanian, etc.) is translated as Grand Prince, whereas most western European languages (English, French, Spanish, Italian etc.) use expressions corresponding to Grand Duke. Unlike a duke, the sovereign grand duke is considered royalty (or in German, 'royal nobility', Königsadel). The proper form of address for a grand duke is His Royal Highness (HRH), whereas for a non-royal duke in the United Kingdom it is His Grace.

In contrast to this, the rank of a duke differs from one country to the next. In Germany, for example, a duke is listed in the aristocratic hierarchy below an emperor (Kaiser), king (König), grand duke (Großherzog), elector (Kurfürst) and sovereign prince (Fürst) – in that order – whereas in Britain the duke comes third after king/queen and prince (there are no British grand dukes or electors).

In all countries, there existed an important difference between "sovereign dukes" and dukes subordinate to a king or emperor. Some historic duchies were sovereign in areas that would become part of nation-states only during the modern era, such as Germany (a federal empire) and Italy (a unified kingdom). In contrast, others were subordinate districts of those kingdoms that had unified either partially or completely during the medieval era, such as France, Spain, Sicily, Naples, and the Papal States. In England, the term is used in respect of non-territorial entities.

...
Wikipedia