The Duke of Lorraine was a title held by the rulers of the Duchy of Lorraine, a historical region in northeastern France. The duchy was established in the 10th century and became a significant political entity in Europe. The dukes played a crucial role in the power struggles between France and the Holy Roman Empire. Notable dukes include Francis I, Duke of Lorraine, who later became Holy Roman Emperor as Francis II. The duchy was eventually annexed by France in the 18th century, but the title of Duke of Lorraine continued to be used by descendants of the ruling family, maintaining its historical significance.