The Duke of Lorraine was a title held by the rulers of the historical region of Lorraine in northeastern France. The duchy emerged in the 10th century and became a significant political entity in medieval Europe. The dukes played a crucial role in regional power struggles and alliances, particularly during the Hundred Years' War and the Thirty Years' War. The most notable duke was Francis I, who later became the Holy Roman Emperor. 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.