King Louis XVI had a troubled reign as France's absolute monarch from 1774 to 1789.
Popular discontent with the royals ousted them from power and Louis XVI and his wife, Marie Antoinette, were eventually executed during the French Revolution in 1793.
Legend had it that at the execution, witnesses dipped their handkerchiefs in the decapitated monarch's blood, and one person apparently put a bloody handkerchief in a decorative gourd as a souvenir.
Though the handkerchief has long-since disintegrated, the blood remained. To identify it, researchers used samples from a head believed to belong to King Henry IV, Louis XVI's direct descendant. "Good King Henry" lived about 200 years before the executed king.
During the French revolution, angry crowds sacked the royal tombs and cut off the embalmed head of Henry IV. It was held privately until 2010.
A 2010 facial reconstruction strongly suggested that the head belonged to French King Henry IV, but initial DNA samples were too badly decomposed for analysis.
A forensic scientist managed to get DNA from Henry's head, and compared it to the blood believed to be Louis XVI's in the decorative gourd. The analysis confirmed the two samples came from related men. The results authenticated that both the head and the blood came from the long-dead French monarchs.