Until 1864, Ærø was part of the Danish Duchy of Schleswig – the area of Schleswig/Southern Jutland is now divided between Denmark (Northern Schleswig) and Germany (Southern Schleswig).
The Treaty of Heiligen was signed in 811 between the Danish King Hemming and Charlemagne, by which the border was established at the Eider.
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7 December – Battle of Bornhöved, part of the War of the Sixth Coalition, is fought Danish troops and Swedish cavalry at the small village of Bornhöft in the Duchy of Schleswig and results in Swedish victory.
Adolph Ernst Kroeger (born Schwabstedt duchy of Schleswig, 28 December 1837; died St. Louis, Missouri, 8 March 1882) was a translator and author who contributed significantly to the understanding of German literature in the United States.
Two wooden figures—the Braak Bog Figures—of "more than human height" were unearthed from a peat bog at Braak in Schleswig, Germany.
Hans Momsen lived on a small farmstead in Fahretoft, Duchy of Schleswig, now a part of the Dagebüll municipality in Germany.
The Legion and the Hanseatischen Bürgergarden went with Count Tettenborn and in Mecklenburg joined the Allied northern army under Wallmoden's command, later taking part in the fighting in the area around Mecklenburg and in parts of the Duchy of Schleswig.
The seat of the duke was Sonderburg, but 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.
Mechthild (1225–1288): she married firstly in 1237 Abel of Denmark, Duke of Schleswig and later King of Denmark (this marriage provided the basis for later claims by the Schauenburgers on the Duchy of Schleswig); and secondly Birger Jarl, Regent of Sweden
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.