John I

No king of France reigned more briefly than John I, whose entire life and rule lasted five days in November 1316. He was the posthumous son of Louis X, who had died in June of that year, and of his second wife, Clémence of Hungary. Because the late king left no son, only a daughter of doubtful standing, the kingdom waited through the summer and autumn for the queen's delivery, with Louis's brother Philip, Count of Poitiers, governing as regent.

The situation was without precedent in the Capetian dynasty, which had passed the crown from father to son in unbroken succession since 987. Louis X's daughter Joan, born of his first marriage to Margaret of Burgundy, was compromised in the eyes of many by the adultery scandal of 1314, in which her mother had been imprisoned, leaving her paternity open to insinuation. A male child of Clémence would resolve the question; the alternative would force the realm to decide, for the first time, whether a woman could inherit the French throne.

John was born on 15 November 1316 and was king from the moment of his birth. He died on 19 or 20 November, before he could be christened with any ceremony of state, and was buried at the abbey of Saint-Denis. He is the only French monarch to have held the crown from birth to death, and the only one named John until his successor's son took the name a generation later.

The infant's death converted the regency into a contested succession. Philip of Poitiers moved quickly, had himself crowned at Reims in January 1317 as Philip V, and secured from an assembly of notables the affirmation that a woman could not succeed to the kingdom of France. Joan was set aside, compensated later with the kingdom of Navarre, which did pass through the female line. The principle established in 1316–1317, subsequently buttressed by appeal to the Salic law, governed the French succession thereafter and underlay the dynastic dispute with England that opened the Hundred Years' War. Decades later a Sienese merchant, Giannino di Guccio, claimed to be the surviving John I, allegedly swapped in the cradle; the claim found little support but attests to the durable strangeness of the five-day reign.