Which European country is the world's only sovereign grand duchy?

Luxembourg is the world’s only sovereign grand duchy.

A grand duchy is a state ruled by a grand duke or grand duchess, a rank historically below an emperor or king but above a duke. Luxembourg became a grand duchy in 1815, when the Congress of Vienna reorganized Europe after the Napoleonic Wars. The title was given to William I of the Netherlands, who was also Luxembourg’s ruler.

For much of the nineteenth century, Luxembourg was linked to the Netherlands in a personal union, meaning the same monarch ruled both countries but they remained distinct states. That arrangement ended in 1890, when a succession rule allowed Adolf of Nassau-Weilburg to become Luxembourg’s grand duke instead of the Dutch queen Wilhelmina.

Other European states, including Tuscany, Baden, and Hesse, were once grand duchies, but their monarchies disappeared or their territories were absorbed into other states. Luxembourg’s survival makes it the sole sovereign grand duchy today. Its monarch is a constitutional head of state, and the country is bordered by Belgium, Germany, and France.