A reign, technically, begins at the moment of the predecessor's death and ends at the moment of the successor's succession. In most of recorded history, a reign of anything under a year is considered exceptionally short; most monarchs reigned for somewhere between fifteen and forty years. The cases below are all measured not in years but in months, days, or, in two cases, minutes. Each of them has a specific reason for having been so brief, and together they form a small catalog of how monarchy can fail, in real time, at its own handover point.
1. Louis XIX of France (approx. 20 minutes, August 2, 1830)
During the July Revolution of 1830, King Charles X of France abdicated on August 2, in an attempt to preserve the Bourbon line by passing the crown directly to his grandson, the Duke of Bordeaux, who would reign as Henri V. Between Charles X's signature on the abdication document and the Duke of Bordeaux's legal accession, however, there was an intermediate step: Charles X's son, Louis-Antoine, the Duke of Angoulême, technically became King Louis XIX during the period required for him to sign his own renunciation in favor of his nephew.
The two signatures occurred, by the most widely-cited account, approximately twenty minutes apart. Louis XIX's "reign" consisted entirely of the time required for his father to hand him the pen, for him to read the document renouncing the throne, and for him to sign it. The rapid sequence of abdications did not, in any case, persuade the provisional government, which installed Louis-Philippe d'Orléans as a constitutional monarch two days later. Louis XIX is, by most calculations, the shortest-reigning king of France, and possibly of any major European state.
2. Crown Prince Luís Filipe of Portugal (approx. 20 minutes, February 1, 1908)
On February 1, 1908, King Carlos I of Portugal and his elder son Luís Filipe, the Prince Royal, were riding in an open carriage through the Terreiro do Paço in Lisbon when two gunmen, republican activists named Alfredo Costa and Manuel Buíça, opened fire. King Carlos was killed almost instantly. Luís Filipe, who was on the rear-facing seat, was struck in the chest by a bullet and mortally wounded but remained technically alive. Under Portuguese succession law, he became King Luís III of Portugal at the moment of his father's death.
Luís Filipe died approximately twenty minutes later, in the carriage, before he could receive any medical treatment. He thus held the throne of Portugal, as a fatally wounded king, for the duration of the ride to the royal palace. His younger brother Manuel II, who had also been wounded but survived, succeeded him. Manuel's own reign was cut short two years later by the Portuguese revolution of 1910. The Portuguese monarchy ended with him.
3. Dipendra of Nepal (approx. 3 days, June 1–4, 2001)
On the night of June 1, 2001, Crown Prince Dipendra of Nepal, during an argument with his family over his choice of bride, produced multiple firearms at a royal dinner in Narayanhity Palace and killed his father King Birendra, his mother Queen Aishwarya, his brother Prince Nirajan, his sister Princess Shruti, and five other royal relatives. He then shot himself. He did not die immediately; he was transported to the Chhauni Military Hospital in a coma.
Under Nepali succession law, Dipendra became king at the moment of his father's death. He technically reigned, in a coma, from the night of June 1 until his own death on June 4, 2001. His uncle Gyanendra, who had not been present at the dinner, became king upon Dipendra's death and was the last King of Nepal before the 2008 abolition of the monarchy. Dipendra's reign is, as far as I know, the only case in modern history of a king who committed a mass murder and then reigned, unconscious, for three days as a matter of legal formality.
4. Lady Jane Grey of England (9 days, July 10–19, 1553)
Jane Grey was sixteen years old, the great-granddaughter of Henry VII, and by most accounts a reluctant participant in the attempt to place her on the throne. King Edward VI, dying of tuberculosis, had been persuaded in his final weeks to alter the succession by excluding his Catholic half-sister Mary and naming Jane, a Protestant, as his heir. Jane was proclaimed queen on July 10, 1553, three days after Edward's death.
Mary, who had not been consulted, raised a military force in East Anglia and entered London to essentially universal popular acclaim on July 19. Jane's Privy Council switched allegiance the same day. Jane was formally deposed and confined to the Tower of London. She was executed seven months later, on February 12, 1554, aged seventeen. She is traditionally called the "Nine Days' Queen," though the formal calculation of her reign varies between nine and thirteen days depending on which end of the proclamation window is used.
5. Edward V of England (2 months, April 9 – June 25, 1483)
Edward V was twelve years old when he became king of England upon the death of his father Edward IV on April 9, 1483. His uncle Richard, Duke of Gloucester, was appointed Lord Protector and was ostensibly arranging Edward's coronation. Instead, on June 25, Parliament was presented with a document (the Titulus Regius) declaring Edward IV's marriage invalid and therefore his children illegitimate. On the strength of this declaration, Richard was proclaimed King Richard III the following day. Edward V's brief reign ended without his ever having been crowned.
Edward and his younger brother Richard, the "Princes in the Tower," were confined to the Tower of London through the summer of 1483 and were not seen again after approximately August of that year. Their fate remains one of the most debated questions in English historical scholarship. The most widely accepted modern reading is that they were murdered, probably on Richard III's orders, probably in late summer of 1483, but the documentary evidence is genuinely unclear and the alternatives (survival, escape, murder by another party) remain defensible.
6. Empress Suiko's successor Empress Kōgyoku → Empress Saimei (restoration)
An odd case from Japanese imperial history: Empress Kōgyoku, who reigned 642–645, abdicated in favor of her brother after the assassination of the Soga regent by her son, the future Emperor Tenji. Her brother Emperor Kōtoku reigned from 645–654. Upon his death, Kōgyoku was brought out of retirement and re-enthroned as Empress Saimei. She thus held the throne twice, with a nine-year gap. Between the two reigns, she was a retired empress with considerable political influence and no formal title. The Japanese imperial chronology records her as a single person but two separate reigns, which means her second reign, at least, started as the continuation of an earlier interruption — a pattern that would be repeated, in modern times, with Japanese Emperor Meiji's grandson.
7. Constantine XI Palaiologos (approx. 4 years, 1449–1453)
Constantine XI is included here not because his reign was especially short by calendar — four years is actually within normal range — but because it was the last reign of the Byzantine Empire. He became Emperor on January 6, 1449, when his predecessor John VIII died. He reigned through four years of increasingly desperate preparations for what was, by then, the obviously inevitable Ottoman siege of Constantinople. He died on May 29, 1453, in the final hours of the siege, reportedly in the streets of the city, fighting in person. His body was never identified with certainty among the dead. His reign ended with the ending of the Roman Empire, which had been continuous, in one form or another, for approximately 1,480 years.
Constantine XI's reign is sometimes omitted from lists like this because it lasted more than four years, but I include it because it represents a different kind of "short" reign: the one that is cut short by the dissolution of the state itself. His is the only entry on this list whose reign ended not because the king died or was deposed, but because the institution of the throne he sat on ceased to exist.
8. Umberto II of Italy (34 days, May 9 – June 12, 1946)
Umberto II became King of Italy on May 9, 1946, when his father Victor Emmanuel III abdicated in an attempt to dissociate the Italian monarchy from its collaboration with Mussolini and strengthen the monarchy's position in the upcoming constitutional referendum. The gambit did not work. On June 2, 1946, Italian voters chose to abolish the monarchy by a margin of approximately 54% to 46%. Umberto went into exile in Portugal on June 13, 1946.
He is known to Italians of a certain generation as Il Re di Maggio, the King of May, after the month he was king for most of. He lived in Portugal for thirty-seven years and died there in 1983. He never returned to Italy. A law forbidding Italian kings and their male descendants from entering the country was not repealed until 2002. Umberto's reign is the shortest that any major modern state has recorded, and the only case I know of in which a kingship was ended by referendum.
What a reign is
Reading these eight together, one notices that a reign is, legally, something like a formal placeholder. It is not always a period of effective rule. Three of the eight above (Louis XIX, Luís Filipe, Dipendra) reigned only in the specific legal sense, without exercising any substantive power. The others reigned under conditions of deposition, minority, siege, or referendum that meant their power was either contested throughout or never really consolidated.
What the list does not show, and what is worth remembering, is that the longest reigns in history (Louis XIV of France at 72 years, Queen Elizabeth II of the United Kingdom at 70, Johann II of Liechtenstein at 70, Rama IX of Thailand at 70) are doing a different kind of work from the very short reigns. A long reign is, among other things, a demonstration that the institution can absorb an individual ruler for a very long time without breaking. A short reign is a demonstration that the institution can fail at its most basic function, which is succession, and can fail in visible and specific ways. Both kinds of reign are, in their own ways, failures or successes of the same system. Louis XIX's twenty minutes and Louis XIV's seventy-two years are, looked at a certain way, two outcomes of the same set of institutional pressures.