A regnal name, regnant name, or reign name is the name used by monarchs and popes during their reigns and subsequently, historically. Since ancient times, some monarchs have chosen to use a different name from their original name when they accede to the monarchy.
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 Charles X Gustav of Sweden. If a monarch reigns in more than one realm, they 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 VI of Scotland and King James I of England.