Oldenburg house members have reigned at various times in Denmark,[a] Norway, Sweden, Iceland, Greece, several northern German states, Russia[b] and the United Kingdom.

While Schleswig for centuries constituted the southernmost region of Denmark, Holstein historically has been the northernmost area within the Holy Roman Empire.

The lands of Schleswig beyond the river remained a fief of the Danish Crown, while Holstein became an integral part of East Francia, the Kingdom of Germany and the Holy Roman Empire.

Glücksburg is a small coastal town on the German southern side of the fjord of Flensburg that divides Germany from Denmark.

[4] In 1564, Christian I's great-grandson, King Frederick II, in re-distributing Schleswig and Holstein's fiefs, retained some lands for his own senior royal line while allocating Glücksburg to his brother Duke John the Younger (1545–1622), along with Sønderborg, in appanage.

[4] John's heirs further sub-divided their share and created, among other branches, a line of Schleswig-Holstein-Sonderburg dukes at Beck (an estate near Minden bought by the family in 1605), who remained vassals of Denmark's kings.

[7] Frederick suffixed the territorial designation to the ducal title he already held, in lieu of "Beck" (an estate the family had, in fact, sold in 1745).

Prince Christian of Schleswig-Holstein-Sonderburg-Glücksburg, the fourth son of Duke Friedrich of Glücksburg, was recognized in the London Protocol of 1852 as successor to the childless King Frederick VII of Denmark.

Christian IX's daughters, Alexandra and Dagmar (as Maria Feodorovna) became the consorts of, respectively, King Edward VII of the United Kingdom and Emperor Alexander III of Russia.

He was the fourth son of Friedrich Wilhelm, Duke of Schleswig-Holstein-Sonderburg-Glücksburg, whose elder brother (and male-line descendants) retained the Glücksburg dukedom.

The Hellenic constitutional monarchy was usurped in a coup d'état by a military junta in 1967 and the royal family fled into exile.

[clarification needed][12] In 1905, Prince Carl of Denmark became Norway's first independent monarch in 518 years, taking the regnal name Haakon VII.

Born into the house of Glücksburg as a prince of Denmark and Greece, he later relinquished these titles and was created Duke of Edinburgh by his father-in-law, King George VI of the United Kingdom.