A grand duchy is a country which has a Grand Duke or a Grand Duchess as head of state. Between the Napoleonic Wars and World War I there were eight grand duchies in Europe: Baden, Finland, Hesse-Darmstadt, Luxembourg, Mecklenburg-Schwerin, Mecklenburg-Strelitz, Oldenburg, and Saxe-Weimar. Today Luxembourg is the only remaining grand duchy. It has been a grand duchy since 1815, when the Netherlands became an independent kingdom and Luxembourg was handed over to the King of the Netherlands, William I.
Luxembourg remained a Dutch dominion until 1890, when King William III of the Netherlands, who was also Grand Duke Guillaume III of Luxembourg, died without leaving a male heir to the Luxembourgian Grand Duchy. The throne then passed to his cousin, Duke Adolf of Nassau who became HRH Grand Duke Adolf I of Luxembourg.