The Dutch king refused a regnal number because it reminded him of cattle
Before his inauguration, Willem-Alexander of the Netherlands had planned to reign as Willem IV. He changed his mind, saying he had lived 46 years as Willem-Alexander and was not a mere number; regnal numerals, he joked, recalled Dutch cattle naming. Choosing a throne name is an ancient habit, and full of quirks.
A regnal name is the name a monarch or pope uses while reigning, usually followed by a Roman numeral to tell them apart from predecessors of the same name. The numbering can split: one man was James VI of Scotland and James I of England at once. The first holder of a name normally gets no numeral until a second comes along, so Elizabeth of England only became Elizabeth I retroactively once Elizabeth II acceded in 1952. Yet some rulers, such as Juan Carlos I of Spain and Haile Selassie I of Ethiopia, used 'the First' from the start.
The practice is ancient. Several Assyrian kings, and apparently some kings of Judah, took throne names, and Egyptian pharaohs carried several names, most often the praenomen. In Ethiopia's Solomonic dynasty many emperors took throne names: Tafari Makonnen became Haile Selassie, 'Power of the Trinity'. Portugal's Sancho I was born Martin and renamed only when his elder brother, the heir, died, because Sancho had a stronger royal pedigree in Iberia. Queen Victoria was christened Alexandrina Victoria.
China built an elaborate system of its own. Rulers' personal names were taboo, and after death sovereigns received temple names ending in words for progenitor or ancestor, plus posthumous names that judged their virtues. From 140 BC, under Emperor Wu of Han, emperors also declared era names to number years. Ming and Qing rulers usually kept a single era name all reign, which is why they are known by it today; Japan still uses the system.
In Nigeria's traditional states, thrones often rotate among huge pools of cousins sharing a founder's surname, so reign names distinguish branches and double as calendars. In Benin, Erediauwa I's royal name was adopted as a surname by his close family, tying their descendants to the era of his reign.
Source: Regnal name
