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A reign is the period of a person's or dynasty's occupation of the office of monarch of a nation (e.g., Saudi Arabia, Belgium, Andorra), of a people (e.g., the Franks, the Zulus) or of a spiritual community (e.g., Roman Catholicism, Tibetan Buddhism, Nizari Ismailism). In most hereditary monarchies and some elective monarchies (e.g., Holy Roman Empire) there have been no limits on the duration of a sovereign's reign or incumbency, nor is there a term of office. Thus, a reign usually lasts until the monarch dies, unless the monarchy itself is abolished or the monarch abdicates or is deposed.
The term of a reign can be indicated with the abbreviation "r." after a sovereign's name, such as the following:
- George VI, King of the United Kingdom (r. 1936–1952)
- Queen Victoria of the United Kingdom reigned from 1837 to 1901.
- King Victor Emmanuel III of Italy reigned from 1900 to 1946.
- Pope John XXIII reigned from 1958 to 1963.
- King Bhumibol Adulyadej of Thailand reigns from 1946 to present.
- Emperor Akihito of Japan reigns from 1989 to present.
A reign can be ended in three ways:
- Emperor Ferdinand I of Austria, King of Hungary and Bohemia, King of Lombardy-Venetia (r. 1835-1848) was abdicated due to his disabilities (feeble-mindedness and epilepsy). His successor, Emperor Franz Josef (r. 1848–1916) reigned for 68 years.
- King Edward VIII of the United Kingdom reigned from January to December 1936 before he abdicated the throne. After his abdication he became known as the Duke of Windsor. No other monarch of the Kingdom of Great Britain (1707–1800), the United Kingdom of Great Britain and Ireland or the United Kingdom of Great Britain and Northern Ireland has ever abdicated, though forced abdications did occur on rare occasions in the Kingdom of England and the Kingdom of Scotland prior to their merger in 1707.
- Queen Wilhelmina of the Netherlands reigned from 1890 to 1948, before abdicating in favor of her daughter, Queen Juliana who did herself. Juliana then reigned until 1980 when she abdicated in favor of her daughter, Queen Beatrix of the Netherlands. Beatrix would follow suit and she abdicated in 2013 in favour of her son and heir apparent, King Willem-Alexander.
- King Constantine II of Greece reigned from 1963 until the abolition of the Greek monarchy in 1973.
- King Humbert II of Italy reigned for only a few weeks in 1946 before the abolition of the Italian Monarchy.
- Tsar Nicholas II of Russia reigned from 1894 to 1917 during the Russian Revolution when he was forced to abdicate and the Russian Empire was overthrown.
- King Louis XVI of France reigned from the 1770s until the overthrow of the monarchy in 1792 and his execution the following year.
- Shah Mohammad Reza Pahlavi of Iran reigned for more than 37 years (1941–1979) until he fled during the Iranian Revolution, which replaced 2500 years of monarchy with the Islamic Republic of Iran. The Shah died the following year and is buried in Cairo.