A grand duchy is a country or territory whose official head of state or ruler is a monarch bearing the title of grand duke or grand duchess.
Prior to the early 1800s, the only Grand duchies in Europe were located in what is now Italy: Tuscany (declared in 1569) and Savoy (in 1696). During the 19th century there were as many as 14 grand duchies in Europe at once (a few of which were first created as exclaves of the Napoleonic empire but later re-created, usually with different borders, under another dynasty). Some of these were sovereign and nominally independent (Baden, Hesse and by Rhine, Mecklenburg-Schwerin, Mecklenburg-Strelitz, Oldenburg, Saxe-Weimar-Eisenach and Tuscany), some sovereign but held in personal union with larger realms by a monarch whose grand-dukedom was borne as a subsidiary title (Finland, Luxembourg, Transylvania), some of which were client states of a more powerful realm (Cleves and Berg), and some whose territorial boundaries were nominal and the position purely titular (Frankfurt).
In the 21st century, only Luxembourg remains a grand duchy.
The only grand duchy still extant is Luxembourg. It regained its independence from Napoleonic France and became a sovereign grand duchy in 1815 by decision of the Congress of Vienna which dealt with the political aftermath of the Napoleonic Wars.
In order to act as a sufficient counterbalance to France, the Congress decided to grant the dignity of grand duke of Luxembourg to the monarch of the newly created United Kingdom of the Netherlands which comprised present-day Netherlands and Belgium. Luxembourg remained in personal union with the crown of the Netherlands until 1890 when William III, King of the Netherlands and Grand Duke of Luxembourg, died without leaving a male heir. He was succeeded on the Dutch throne by his daughter Wilhelmina, but she could not become Grand Duchess of Luxembourg under the semi-Salic law established by the Congress of Vienna. In terms of the law, the grand ducal throne had to be passed to a male dynast.