Duchy of Holstein-Gottorp

Holstein-Gottorp or Schleswig-Holstein-Gottorp is the historiographical name, as well as contemporary shorthand name, for the parts of the duchies of Schleswig and Holstein, also known as Ducal Holstein, that were ruled by the dukes of Schleswig-Holstein-Gottorp. Other parts of the duchies were ruled by the kings of Denmark. The territories of Gottorp are located in present-day Denmark and Germany. The main seat of the dukes was Gottorf Castle in the city of Schleswig in the duchy of Schleswig. It is also the name of the ducal house, which ascended to several thrones. For this reason genealogists and historians sometimes use the name of Holstein-Gottorp for related dynasties of other countries.

The formal title adopted by these rulers was "Duke of Schleswig, Holstein, Dithmarschen and Stormarn", but that title was also used by his kinsmen, the kings of Denmark and their cadet branches, as it was the common property of all these agnates. The Gottorp branch held Landeshoheit (sovereignty) over the duchy of Holstein in the Holy Roman Empire and over the duchy of Schleswig in the kingdom of Denmark. The name Holstein-Gottorp is for the sake of convenience used instead of the technically more correct "Duke of Schleswig and Holstein in/at Gottorp".

The oldest of the ducal titles was that of Schleswig, which had been confirmed in fief to a royal kinsman by the regent Queen Margaret I of Denmark, Sweden and Norway on behalf of her son Olaf II of Denmark in 1386. The kings of Denmark were granted Holstein as an imperial fief by the Holy Roman Emperor Frederick III from 1474.

In 1544 the so-called "one-third duchy" was ceded to Adolf, third son of King Frederick I of Denmark and the youngest half-brother of King Christian III of Denmark. Thus the surviving House of Holstein-Gottorp is a cadet branch of the House of Oldenburg. The Dukes of Holstein-Gottorp shared the uneasy rule of Schleswig and Holstein with the Kings of Denmark. As such, they were often allies (practically clients) of the Swedes, enemies of the Danes. This longtime alliance was sealed by several dynastic marriages: Christina of Holstein-Gottorp married Charles IX of Sweden, Hedwig Eleonora of Holstein-Gottorp married Charles X Gustavus, Duke Frederick IV married the eldest daughter of King Charles XI of Sweden, and ultimately Prince Adolf Frederick of Holstein-Gottorp ascended to the Swedish throne in 1751, founding the Holstein-Gottorp dynasty of Sweden (ruled 1751–1818).

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