Regnal number

Monarchs with the same given name are distinguished by their ordinals:

• Kings Umberto I and Umberto II of Italy
• Empresses Catherine I and Catherine II of Russia
• Princes Rainier II and Rainier III of Monaco
• Popes Benedict XV and Benedict XVI

Ordinals may also apply where a ruler of one realm and a ruler of that realm's successor state share the same name:

• Queens Elizabeth I of England and Elizabeth II of the United Kingdom.
• Kings Alfonso XI of Castile and Alfonso XII of Spain.
• Kings Victor Emmanuel I of Sardinia and Victor Emmanuel II of Italy

Almost all West European monarchs and popes after medieval times have used ordinals. Ordinals are also retrospectively applied to earlier monarchs in most works of reference, at least as far as they are not easy to distinguish from each other by any other systematical means. In several cases, various sorts of "semi-regnal" members of dynasties are also numeraled, to facilitate their individuality in works of reference – in cases such as co-regents, crown princes, succession-conveying consorts, prime ministers and deputy monarchs. In the first centuries after the Middle Ages, the use was sometimes sporadic, but became established by the 18th century. In the past couple of centuries, European monarchs without an official ordinal have been rarities.

As a rule of thumb, medieval European monarchs did not use ordinals at their own time, and those who used were rarities and even their use was sporadic. Ordinals for monarchs before the 13th century are anachronisms, as are also ordinals for almost all later medieval monarchs. Still, they are often used, because they are a practical way of distinguishing between different historical monarchs who had the same name.

Popes were apparently the first to assume official ordinals for their reigns, although this occurred only in the last centuries of the Middle Ages. It is clear, from renumberings of Popes John XV–XIX and Popes Stephen II–IX, that as of the 11th century the popes did not yet use established ordinals. The official, self-confirmed numbering of John XXI means that at latest from the 13th century the popes did take official ordinals in their accession.

Emperor Frederick II, king Charles II of Naples and king Premislas II of Poland evidently used ordinals sometimes during their reign, whereas most of their contemporary monarchs did not. In the 14th century, Emperor Charles IV sometimes used that ordinal. Presumably, use of the ordinal of king Frederick III of Sicily also is contemporaneous. The royal chroniclers of the Abbey of Saint-Denis were using ordinals to refer to the French kings as early as the thirteenth century with the practise entering common usage among royalty and the nobility by the late fourteenth century. The British tradition of consistently and prevalently numbering monarchs dates back to Henry VIII and Mary I; however, sporadic use occurred at least as early as the reign of Edward III.

The long history of the papacy has led to difficulties in some cases. For example, Stephen was only pope for three days before dying of apoplexy, and was never consecrated. Because not all list-makers count him as having been pope (as Stephen II), there has been some confusion in regard to later popes who chose the name Stephen. Later Stephens are sometimes numbered with parentheses, e.g., his immediate successor (in name) is denoted either Stephen (II) III or Stephen III (II). The church did consider Stephen II a pope until 1960, when he was removed from the list of popes in 1961. The history of the numbering of popes taking the regnal name "John" is even more convoluted, owing to the long history of popes taking the name (a common name, chosen frequently to honour the Apostle), bad record-keeping, and political confusion; among other results, the regnal name "John XX" is completely skipped under all reckonings.

In the case of personal unions, some monarchs have had more than one ordinal, because they had different ordinals in their different realms. For instance, Charles XV of Sweden was also king of Norway, but in Norway he went under the name Charles IV. The Swedish-Norwegian union was in force 1814–1905 and both realms had had kings called Charles before the union, but Sweden had had more kings by that name.

In the event of one kingdom achieving independence from another but retaining the same monarch, the monarch often retains the same number as was already used in the older realm. King Christian X of Denmark thus became King Christian X of Iceland when Iceland became an independent kingdom in personal union with Denmark in 1918. The same is true for Commonwealth realms, where the monarch retains the regnal number from the British line of monarchs (see below).

In some monarchies it is customary not to use an ordinal when there has been only one holder of that name. For example, Queen Victoria will not be called Victoria I unless there is a Victoria II. This tradition is applied in the United Kingdom, Belgium, Luxembourg, Norway and the Netherlands. It was also applied in most of the former German monarchies and in Hungary. In Sweden, the practice is not consistent, as Sigismund and Adolf Frederick never have ordinals, whereas Frederick I often does.

Other monarchies assign ordinals to monarchs even if they are the only ones of their name. This is a more recent invention and appears to have been done for the first time when Francis I of France issued testoons (silver coins) bearing the legend FRANCISCVS I DE. GR. FRANCORV. REX. This currently is the regular practice in Spain and Monaco (at least for Prince Albert I, as Princess Louise Hippolyte, who reigned 150 years earlier, does not appear to have used an ordinal). It was also applied in Brazil, Greece, Italy, Mexico, Montenegro, Portugal (where Kings Joseph, Louis and Charles are usually referred to as "Joseph I", "Louis I" and "Charles I" although there has not yet been any Joseph II, Louis II or Charles II, but Kings Denis, Edward, Sebastian and Henry are usually referred without the ordinal). The ordinal for King Juan Carlos I of Spain is used in both Spanish and English, but he is sometimes simply called King Juan Carlos of Spain in English. In Russia, use of "The First" ordinal started with Paul I of Russia. Before him, neither Anna of Russia nor Elizabeth of Russia had the "I" ordinal. In Ethiopia, Emperor Haile Selassie used the "I" ordinal (Ge'ez: ቀዳማዊ, qädamawi) although previous Ethiopian monarchs had not used it, and they are not referred as "the first" unless there were successors of the same name.

The Catholic papacy used the ordinal I under Pope John Paul I, but early popes who are the only ones to have reigned under a certain name are not referred to as "the first" (for instance, Peter the Apostle; his immediate successor, Pope Linus, as well as Pope Anacletus, are referred to without an ordinal). The current Pope Francis has declined the use of an ordinal, but on the other hand, his Orthodox counterpart, Patriarch Bartholomew I of Constantinople, uses one, as does Aram I, the catholicos of the Armenian Apostolic Church.

In Austria, Emperors Francis, Ferdinand, Francis Joseph and Charles all styled themselves as "the first" although all the only Emperors of Austria with those names. Three of those names were previously the names of Austrian Archdukes (the Archduchy of Austria was a state within the Holy Roman and the Austrian Empires), which makes three of these emperors Francis II, Ferdinand V, and Charles IV in their capacity as Archdukes. Francis Joseph was the first Austrian Archduke of that name.

The use of "The First" ordinal is also common to self-proclaimed ephemeral "kings" or "emperors", such as Napoleon I in France; Dessalines, Christophe and Soulouque in Haiti; Iturbide in Mexico; Zog in Albania; Bokassa in the Central African Empire; Skossyreff in Andorra; Theodore in Corsica; and "Emperor" Norton in San Francisco. In those cases, they wanted to emphasize the change of regime they introduced or attempted to introduce.

It is traditional amongst French monarchists to continue to number their pretenders even though they have never reigned. Hence, a supporter of the late Comte de Paris would have referred to him as Henri VII even though only four men named "Henri" have been King of France.

Non-consecutive ordinals may indicate dynastic claims for non-regnant monarchs. For example, after Louis XVI of France was executed during the French Revolution, legitimists consider him to have been succeeded by his young son, whom they called Louis XVII. Although the child died in prison a few years later and never reigned, his uncle, who came to the French throne in the Bourbon Restoration, took the name Louis XVIII in acknowledgement of his dynasty's rights. Similarly, after Emperor Napoleon I's regime collapsed, he abdicated in favour of his four-year-old son, who was proclaimed Napoleon II. The young emperor was deposed only weeks later by Napoleon's European rivals and was never recognized internationally; but when his first cousin Louis Napoleon Bonaparte proclaimed himself Emperor in 1852, he declared himself Napoleon III of France in recognition of his predecessor.

While reigning monarchs use ordinals, ordinals are not used for royal female consorts. Thus, while King George V used an ordinal to distinguish him from other kings in the United Kingdoms called George, his wife, Queen Mary, had no ordinal.

The lack of an ordinal in the case of royal consorts complicates the recording of history, as there may be a number of consorts over time with the same name with no way to distinguish between them. For that reason, royal consorts are sometimes after their deaths recorded in history books and encyclopaedias by the use of their premarital name or, if they were from royalty or sovereign nobility, the name of the dynasty or the country. For example, Henry VIII of England's fifth wife, Catherine Howard (of noble but not sovereign ancestry), is known by her maiden surname, and George V's wife (a descendant of the sovereign ducal house of Württemburg) is commonly known as Mary of Teck (after her father's title) and Edward VII's wife (a daughter of the King of Denmark) is known as Alexandra of Denmark.

It is not usual to number male consorts of a queen regnant or even to consider them as kings, unless both of them are crowned, as was the case of Mary II and William III of England, or of all male kings consort in the medieval crusader Kingdom of Jerusalem and in the medieval Spanish Kingdom of Navarre.

In the history of Spain, the only king consort was Francisco de Asis de Borbón, the husband of Isabella II, but he is not numbered at all and is never included in official regnal lists.

The Kingdom of Portugal had the uncommon custom of recognizing, and numbering, a reigning queen's consort as king only if he fathered an heir apparent to her; even so they are not included in official lists of Portuguese heads of state. Thus, the husband of Maria I was known as King Consort Pedro III until his death in 1786, before his wife, and the husband of Maria II was also titled King Consort Fernando II when their elder son, the future King Pedro V, was born in 1837, but that title was retired from him when his wife died in 1853.