The northernmost state of the Holy Roman Empire and, after 1815, a member of the German Confederation — yet ruled for its whole existence by the Danish House of Oldenburg in personal union with the Danish fief of Schleswig. Its German status inside a Danish monarchy is the hinge of the Schleswig-Holstein Question.
Danish rights renounced at the Treaty of Vienna, 30 October 1864; administered by Austria under the 1865 Gastein Convention and annexed by Prussia after the Austro-Prussian War of 1866, becoming the southern half of the Province of Schleswig-Holstein.