Duke of Schleswig-Holstein-Sonderburg-Plön

The Duchy of Schleswig-Holstein-Sonderburg-Plön (German: Herzogtum Schleswig-Holstein-Sonderburg-Plön), also Schleswig-Holstein-Plön, Holstein-Plön or just Duchy of Plön, was a small sub-duchy (Teilherzogtum) created by the physical division of the Duchy of Schleswig-Holstein-Sonderburg. Today, its remaining significance is primarily the building of Plön Castle. The Duchy of Plön was not a territorial dukedom in its own right, but a sub-division within the state structure of the duchies of Schleswig and Holstein. The scattered territorial dominion lay mostly in the southeast part of present-day German state of Schleswig-Holstein.

The duchy was created on the death of Duke John III from the House of Schleswig-Holstein-Sonderburg in 1622 as the result of a provision in his will. This provision was in disregard of the Treaty of Ripen that prohibited any arbitrary division of land in Schleswig-Holstein. The Dukes of Plön were so-called Abgeteilte Herren whose status was not based on territorial lordship. In addition to the creation of Schleswig-Holstein-Plön, the import of this will and testament was that four other small duchies were formed as part of the physical division of Schleswig-Holstein.

The first Duke of Plön had already received a share of the Duchy of Schleswig-Holstein-Ærø in 1633, which he immediately sold to his brothers, who thereby increased their originally equal shares in the island of Ærø. In 1669 the Duchy of Schleswig-Holstein-Plön was given the land of another ducal sub-division, that of Schleswig-Holstein-Sonderburg-Norburg, consisting of the northern part of the island of Als. This part of the duchy, remote from Plön, was spun off as the line of Plön-Norburg in the wake of further territorial divisions, but was reunited with Plön again in 1706. In 1671 there was a further division of assets after the death of Duke Joachim Ernest, and the Duchy of Rethwisch was created around the place of the same name under his second son, Duke Joachim Ernest; this reverted to the line of the first born in 1729.

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