The Grand Duke of Luxembourg is the monarchial head of state of Luxembourg. Luxembourg has been a grand duchy since 15 March 1815, when it was elevated from a duchy, and was in personal union with the Netherlands until 1890. Since 1815, there have been nine monarchs of Luxembourg, including the incumbent, Henri. Luxembourg is the world's only sovereign extant Grand Duchy, a status to which Luxembourg was promoted in 1815 upon its unification with the Netherlands under the House of Orange-Nassau.