Duchies in Sweden

Duchies in Sweden have been allotted since the 13th century to powerful Swedes, almost always to princes of Sweden (only in some of the dynasties) and wives of the latter. From the beginning these duchies were often centers of regional power, where their dukes and duchesses had considerable executive authority of their own, under the central power of their kings or queens regnant. Since the reign of King Gustav III the titles have practically been nominal, with which their bearers only rarely have enjoyed any ducal authority, though often maintaining specially selected leisure residences in their provinces and some limited measure of cultural attachment to them.

In Sweden today, Duke (hertig) is considered a royal title, and is only given to members of the Royal House (currently Bernadotte). Unlike British duchies, for example, these Swedish titles are not hereditary. Modern Swedish duchies have always been named for the historical provinces of Sweden, which are no longer governmental entities. Currently, there are eight such duchies one of which includes two of the provinces:

The titles today are given to, and kept by, legitimate members of the Swedish royal family for life, except for Swedish monarchs, who do not continue to hold ducal titles after ascending the throne. Only in connection with his ascension in 1973 has the current king occasionally been referred to as King of Sweden and Duke of Jämtland. However, his wife, current Queen Silvia, whom he married in 1976, is not a duchess, and no other queen consorts have ever continued to have any such title either, after their husbands became King. Otherwise, royal spouses of ducal title holders are also created dukes and duchesses upon marriage (this would not include spouses who do not become Swedish royalty, such as those who married former dukes who had given up their titles for unapproved marriages). The first example of a man acquiring the Swedish ducal title of a woman was at the 2010 marriage of Crown Princess Victoria to Prince Daniel. Currently the prerequisite for a ducal title has been assumed by the public to be the position of Prince or Princess of Sweden, and for that being a Swedish citizen, however no definite policy has been published.

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Wikipedia