The Grand Duchy of Luxembourg was created in 1815 by the Congress of Vienna and given to the new King of the United Kingdom of the Netherlands in exchange for his ancestral Principality of Orange-Nassau, which went to Prussia; as a result, the first Grand Dukes of Luxembourg were Dutch kings also, and their heirs were eligible for both thrones.
This union began to crack in 1884, when the last son of the King-Grand Duke died, leaving no male heir in the Orange-Nassau line.
[1] This branch would face a similar situation only twenty years later: the Grand Duke had six daughters but no sons, and had deemed his cousins the Counts of Merenberg, issue of a morganatic marriage, illegible to inherit the throne.
Instead (to the protest of the Count of Merenberg),[2] he instituted a solution, whereby his daughters (and their male heirs) became eligible to succeed.
[3] In 2011 the law of succession was changed to allow women to inherit the throne with equal right to men.