A Regnal Name, or Reign Name, is the name used by Monarchs and Pope's during their reigns. The practice dates back many thousands of years and interestingly some Monarchs have chosen to use a completely different name from their original name when they accede to the Throne.
The Regnal Name is usually followed by a Regnal Number, written as a Roman numeral, to differentiate that monarch from others who have used the same name while ruling the same realm. In some cases, the Monarch has more than one Regnal Name, but the Regnal Number is based on only one of those names, for example King Charles X Gustav of Sweden, King George Tupou V of Tonga. If a Monarch Reigns in more than one Realm, He or She may carry different ordinals in each one, as some realms may have had different numbers of rulers of the same Regnal Name. For example, the same person was both King James I of England and King James VI of Scotland.
The ordinal is not normally used for the first ruler of the name, but is used in historical references once the name is used again. Thus, Queen Elizabeth I of England was called simply "Elizabeth of England" until the accession of Queen Elizabeth II of England almost four centuries later in 1952; subsequent historical references to the earlier queen retroactively refer to her as Elizabeth I of England. However, Tsar Paul I of Russia, King Umberto I of Italy, King Juan Carlos I of Spain, Emperor Haile Selassie I of Ethiopia and Pope John Paul I all used the ordinal I (first) during their reigns, while Pope Francis does not. In spoken English, such names are pronounced as "Elizabeth the First", "George the Sixth", etc.
In some countries in Asia, Monarchs took or take era names. While era names as such are not used in many Monarchies, sometimes eras are named after a monarch (usually long-lived), or a succession of Monarchs of the same name. This is customary; there is no formal or general rule. For example, the whole period during which a succession of four Georges (King George I, George II, George III and George IV of Great Britain) of the Hanoverian dynasty reigned in Great Britain became known as the Georgian Era. Conversely, although there were many Edwards, the Edwardian Era always refers to the reign of King Edward VII of England at the beginning of the 20th century.