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 Sønderborg. 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.
The dukes of Schleswig-Holstein-Sonderburg repeatedly split their possessions among their heirs, so that a variety of very small territories came into existence.
After the death of Duke John in 1622, the duchy was divided among those sons who were legal heirs and the House of Schleswig-Holstein-Sonderburg produced several collateral lines. The names of the individual lines added the name of their respective Residenz town to the family line. The sons of Duke Alexander - a son of Duke John - also received or acquired some territories outside of Schleswig-Holstein to sustain themselves.
Some of the newly created sub-duchies only had a few square kilometres of land and their masters were merely titular dukes. These new lines sometimes only lasted for a short time before their estates passed to other lines as a result of inheritance or bankruptcy, or even went back to the Danish royal house.
In 1668, the king of Denmark confiscated Sonderburg because of excessive debt. Several of the lines that had split off from Sonderburg continued to exist, however. From the money that was left after all debts were paid, the last duke of Sonderburg bought properties in Franzhagen near Schulendorf; henceforth the line was known as Franzhagen.
The dukes of Schleswig-Holstein-Sonderburg form a line of the House of Oldenburg; it consists of the male-line descendants of Duke John the Younger. The current royal houses of Denmark and Norway belong to the Schleswig-Holstein-Sonderburg line.
