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An interregnum (plural interregna or interregnums) is a period of discontinuity or "gap" in a government, organization, or social order. Archetypally, it was the period of time between the reign of one monarch and the next (coming from Latin inter-, "between" and rēgnum, "reign" [from rex, rēgis, "king"]), and the concepts of interregnum and regency therefore overlap. Historically, the longer and heavier interregna were typically accompanied by widespread unrest, civil and succession wars between warlords, and power vacuums filled by foreign invasions or the emergence of a new power. A failed state is usually in interregnum.
The term also refers to the periods between the election of a new parliament and the establishment of a new government from that parliament in parliamentary democracies, usually ones that employ some form of proportional representation that allows small parties to elect significant numbers, requiring time for negotiations to form a government. In the UK, Canada and other "first past the post" electoral systems, this period is usually very brief, except in the rare occurrence of a hung parliament as occurred both in the UK and in Australia in 2010. In parliamentary interregnums, the previous government usually stands as a caretaker government until the new government is established.
The term has been applied to the period of time between the election of a new President of the United States and his or her inauguration, during which the outgoing president remains in power, but as a lame duck. In some Christian churches, "interregnum" describes the time between vacancy and appointment of priests or bishops to various roles.
Historical periods of interregnum
Particular historical periods known as interregna include:
- The period of 206–202 BC in China, after the death of the final Qin emperor, when there was a contest to the throne. It ended with the accession of Liu Bang, ushering in the Han Dynasty.
- The Crisis of the Third Century (235–284) in the Roman Empire, when it was split into multiple realms and chaos (invasion, civil war, Cyprian Plague, and economic depression) was a constant threat until Aurelian and Diocletian restored the empire.
- The ten-year period from 574/575 to 584/585 in the Kingdom of the Lombards, known as the Rule of the Dukes
- The 1022–1072 period in Ireland, between the death of Máel Sechnaill mac Domnaill and the accession of Toirdhealbhach Ua Briain, is sometimes regarded as an interregnum, as the High Kingship of Ireland was disputed throughout these decades. The interregnum may even have extended to 1121, when Toirdhealbhach Ua Conchobhair acceded to the title.
- From 13 April 1204 to 25 July 1261 in the Byzantine Empire. Following the Sack of Constantinople during the Fourth Crusade, the Byzantine Empire was dissolved, to be replaced by several Crusader states and several Byzantine states. It was re-established by Nicean general Alexios Strategopoulos who placed Michael VIII Palaiologos back on the throne of a united Byzantine Empire.
- From 21 May 1254 to 29 September 1273, The Great Interregnum in the Holy Roman Empire after the deposition of the last Hohenstaufen emperor Frederick II by Pope Innocent IV and the death of his son King Conrad IV of Germany until the election of the Habsburg scion Rudolph as Rex Romanorum.
- First Interregnum in Scotland, which lasted from either 19 March 1286 or 26 September 1290 until 17 November 1292. The exact dating of the interregnum depends on whether the uncrowned Margaret, Maid of Norway was officially queen before her death in 1290. It lasted until John Balliol was crowned King of Scots.
- Second Interregnum in Scotland, from 10 July 1296, when John Balliol was deposed, until 25 March 1306, when Robert the Bruce was crowned.
- From 14 January 1301 until 1308 in the Kingdom of Hungary between the extinction of the Árpád dynasty and when Charles I of Hungary secured the throne for himself against several pretenders.
- From 5 June 1316 to 15 November 1316 in France and Navarre, between the death of Louis X and the birth of his posthumous son John I.
- From 2 August 1332 until 21 June 1340 in Denmark when the country was mortgaged to a few German counts.
- The Portuguese Interregnum, from 22 October 1383 until 6 April 1385, a result of the succession crisis caused by the death of Ferdinand I without a legitimate heir.
- The Ottoman Interregnum, from 8 March 1403 until 1413, a result of the death of Sultan Bayezid I at the hands of Central Asian warlord Timur. A period of civil war ensued as none of Bayezid's sons could establish primacy. The crises was resolved when one of his sons, Mehmed, defeated and killed his brothers and reestablished the Empire.
- From 20 January 1410 to 1412 in the Kingdom of Aragon. The death of King Martin without heir led to a succession crisis and a period of civil war, resolved ultimately by the Compromise of Caspe.
- The 1453–1456 period of civil war in Kingdom of Majapahit (now in Java, Indonesia)
- From 1481 until 1483 in the Kingdom of Norway, during a conflict over the succession of John, during the period of the Kalmar Union. The Norweigian Council of the Realm initially refused to accept the hereditary succession of John; as they asserted that Norway was an elective monarchy. When no serious opposition candidate emerged, the Council relented and elected John.
- From 6 April 1490 to 15 July 1490 in the Kingdom of Hungary between the death of Matthias Corvinus and election of Vladislaus II.
- From 6 April 1490 to 1492 in the Kingdom of Croatia. The estates of Croatia declined to recognise Vladislaus II until he had taken oath to respect their liberties. 
- The Time of Troubles in Russia (1598–1613) between the Rurikid and Romanov dynasties.
- The Interregnum (1649–1660) was a republican period in the three kingdoms of England, Ireland and Scotland. Government was carried out by the Commonwealth and the Protectorate of Oliver Cromwell after the execution of Charles I and before the restoration of Charles II
- A second English interregnum occurred between 23 December 1688, when James II was deposed in the Glorious Revolution, and the installation of William III and Mary II as joint sovereigns on 13 February 1689 pursuant to the Declaration of Right.
- The brief Russian interregnum of 1825, caused by uncertainty over who succeeded the deceased Emperor Alexander I, only lasted between between 1 December and 26 December 1825, but was used to stage the highly resonant Decembrist revolt.
In some monarchies, such as the United Kingdom, an interregnum is usually avoided due to a rule described as "The King is dead. Long live the King", i.e. the heir to the throne becomes a new monarch immediately on his predecessor's death or abdication. This famous phrase signifies the continuity of sovereignty, attached to a personal form of power named Auctoritas. This is not so in other monarchies where the new monarch's reign begins only with coronation or some other formal or traditional event. In the Polish-Lithuanian Commonwealth for instance, kings were elected, which often led to relatively long interregna. During that time it was the Polish primate who served as an interrex (ruler between kings). In Belgium the heir only becomes king upon swearing before the parliament.
Japanese era names
While not describing true interregna, the Japanese era name or nengō system which was introduced in reign of Emperor Kōtoku was abandoned at the end of his reign, thus resulting in sitting emperors without era names; these era names were not updated for some time, except for a very brief re-occurrence near the close of Emperor Temmu's reign.
During the nearly half-century after Emperor Kōtoku, the reigning sovereigns were
- Saimei-tennō (斉明天皇)
- Tenji-tennō (天智天皇)
- Kōbun-tennō (弘文天皇)
- Temmu-tennō (天武天皇)
- Jitō-tennō (持統天皇)
- Mommu-tennō (文武天皇).
The first year of Emperor Mommu's rule (文武天皇元年; 686) could be arguably abbreviated as "the first year of Mommu" (文武元年; 686), but this is nowhere understood as a true era name. The reigns of Japanese emperors and empresses were not considered to also be the same as the era name until the Meiji era. References to the emperors of Japan who ruled during this period are properly written as, for example, "the 3rd year of Emperor Mommu" (文武天皇３年; Mommu is the emperor's name, not that of the era), and not "the 3rd year of Mommu" (文武３年; this second writing implies that Mommu is the era name).
The two periods in the pre-Taihō years without era names are 654 (the end of the Hakuchi era) through 686 (the reinstatement of the Shuchō era), and again from 686 (the Shuchō era) to 701 (some time in the middle of the reign of Emperor Mommu), when the Taihō era was declared and nengō reinstated.
Papal interregnum (or sede vacante)
An interregnum occurs also upon the death or resignation of the Pope, though this is generally known as a sede vacante (literally "vacant seat"). The interregnum ends immediately upon election of a new Pope by the College of Cardinals.
"Interregnum" is the term used in the Anglican Communion to describe the period before a new parish priest is appointed to fill a vacancy. During an interregnum, the administration of the parish is the responsibility of the churchwardens.
- 1946–1948 — Men's World Chess Champion Alexander Alekhine died of natural causes in 1946. Interregnum lasted until 1948, when Mikhail Botvinnik won a FIDE-held chess tournament to decide on a successor.
- 1944–1950 — Women's World Chess Champion Vera Menchik killed in an air-raid during World War II in Britain in 1944. Interregnum lasted until 1950, when Lyudmila Rudenko won a FIDE-held chess tournament to decide on a successor.
The events of Isaac Asimov's Foundation Trilogy take place during the galactic interregnum in his Foundation Universe, taking place in the 25th millennium. Foundation begins at the end of the Galactic Empire and notes in the novels from the Encyclopedia Galactica imply that a Second Galactic Empire follows the 1000 year interregnum.
The Old Kingdom Trilogy takes place after 200 years of interregnum, where the reigning Queen and her two daughters were murdered by Kerrigor, 180 years of regency first and 20 years of anarchy following the death of the last Regent.
The Vlad Taltos series is set in a fantastical world of magic, at a time directly following a 250-year interregnum wherein traditional sorcery was impossible due to the orb being destroyed.
In the world of the Elder Scrolls, there was an Interregnum in the Second Era when the Second Cyrodillic Empire collapsed. It led to just over four centuries of bickering between small kingdoms and petty states. The Interregnum ended when Tiber Septim, or Talos, formed the Third Empire after a decade of war.
- Giorgio Agamben
- Geoffrey of Monmouth
- Imperial Vicar
- Interrex (Poland)
- Presidential transition
- On the Way Out: Interregnum Presidential Activity
- Robert William Seton-Watson:The Southern Slav Question and the Habsburg monarchy page 19
- "Responsibilities and Duties of the Churchwarden". www.churchwardens.com. Retrieved 15 March 2015.