Duchy of Bouillon

The Duchy of Bouillon () (), was a small state located between Luxembourg, Champagne, and the Three Bishoprics and centered on Bouillon. The duchy was claimed by the House of La Tour d'Auvergne. It was officially a State of the Holy Roman Empire. Even though it was a fairly small "state" its capital was the city of Bouillon, now in modern Belgium. The La Tour d'Auvergne family claimed to rule the state from 1456? – 1794 as during the French Revolution it officially changed its legal status to the "". The heir to the ducal throne was traditionally titled the Prince of Turenne.
Dukes of Bouillon.
The dukes were usually addressed by the style of "Serene Highness". 
Jacques Léopold was the last Duke of Bouillon, and following the Napoleonic Wars the duchy was absorbed into the Grand Duchy of Luxembourg.