Schleswig-Holstein-Sonderborg

Schleswig-Holstein-Sonderburg was the name of a branch line of the House of Oldenburg as well as the name of their land. It existed from 1564 until 1668 and was a titular duchy under the King of Denmark, rather than a true territorial dukedom in its own right. The seat of the duke was Sonderburg. Parts of the domain were located in Denmark (in the Duchy of Schleswig), mainly on the islands of Als and Ærø and around Glücksburg, whilst other lands were part of the Holy Roman Empire (in the Duchy of Holstein), including the Ämter of Plön, Ahrensbök, and Reinfeld. As a result of various inheritance arrangements it fragmented into numerous small territories which were eventually absorbed into Greater Denmark in the 18th century.

The ducal family was related to the House of Schleswig-Holstein-Gottorp; both belonged to the House of Oldenburg. The duchy was created in the 16th century when King Frederick II of Denmark shared his part of the duchies of Schleswig and Holstein with his two brothers, each receiving a third of the royal estate in Schleswig and Holstein. Sonderburg was the portion received by Duke John III, called "the Younger". His domain included inter alia the territories of Sonderburg, Norburg, Ærø, Plön and Ahrensbök together with their assigned Ämter or administrative offices. However, the division was not recognized by the local nobles, who considered it illegal, and the new duchy was managed by Duke John as a so-called abgeteilter Herr, i.e. a ruler who did not have the consent of his local landlords. So, while the Duke of Sonderburg received the ducal title and the income from the territory assigned to him, he did not have political rights in this territory. Sovereignty remained with the King of Denmark in his role as Duke of Schleswig-Holstein.

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