It is common to start counting either since the beginning of the monarchy, or since the beginning of a particular line of dynastic succession. For example, Boris III of Bulgaria and his son Simeon II were given their regnal numbers because the medieval rulers of the First and Second Bulgarian Empire were counted as well, although the Saxe-Coburg dynasty only dated back to 1887 and had only distant connections to the previous monarchies. On the other hand, the kings of England were counted starting with the Norman Conquest. That is why the son of Henry III of England is counted as Edward I, even though there were three Edwards before the Conquest.
In any case, it is usual to count only the monarchs or heads of the family, and to number them sequentially up to the end of the dynasty. Sometimes, such as in the case of the Swedish kings, mythical or semi-mythical persons are included. A notable exception to this rule is the German House of Reuss. This family has the particularity that every male member during the last centuries was named Heinrich, and all of them - not only the head of the family - were numbered. While the member of the elder branch were numbered in order of birth until the extinction of the branch in 1927, the members of the younger line were (and still are) numbered in sequences which began and ended roughly as centuries began and ended. This explains why the current head of the Reuss family is called Heinrich IV, his son Heinrich XIV and his son Heinrich XXIX.
Monarchs with the same given name are distinguished by their ordinals:
Ordinals also apply where the historical realms differ:
Almost all monarchs and popes after mediaeval 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 consort, prime ministers and deputy monarchs. In the first centuries after Middle Ages, the use was sometimes sporadical, but became established until 18th century. In the past couple of centuries, European monarchs without an official ordinal have been rarities.
Popes were apparently the first to assume official ordinals for their reigns, although this occurred only in last centuries of the Middle Ages.
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 sporadical. Ordinals for monarchs before 13th century are actually anachronisms, as are also ordinals for almost all later medieval monarchs, too.
It is quite clear, concluded from renumberings of Popes John XV-XIX and Popes Stephen II-IX, that at 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 14th century, Emperor Charles IV used sometimes that ordinal. Presumably, use of the ordinal of king Frederick III of Sicily also is contemporaneous. 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 length of time that the Catholic Church and the Papacy have existed have 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 II (III). The church did consider Stephen II a Pope until 1960, when he was removed from the list of Popes in 1961. Another example of this is that there has never been a Pope John XX.
In some monarchies it is customary not to use an ordinal when there has been one holder of that name. For example, Queen Victoria of the United Kingdom is not called Victoria I except in formal legal documents. This tradition is applied in the United Kingdom, the Netherlands, Luxembourg and Norway. It was also applied in most of the German monarchies. In Sweden, the practice is not consistent, as Sigismund and Adolf Frederick never have ordinals, whereas Frederick I often does.
Other monarchies do assign ordinals to monarchs who are the only ones of their name. This is a more recent invention and appears to be done for the first time when King Francis I of France issued testoons (silver coins) bearing the legend FRANCISCVS I DE. GR. FRANCORV. REX. This currently is the regular practice in Belgium, Spain and Monaco (at least for Prince Albert I, as Princess Louise Hippolyte, who reigned 150 years earlier, doesn't appear to have used an ordinal). It was also applied in Albania during the reign of King Zog, Brazil, Italy, Mexico, Montenegro, Portugal, and by the Papacy under Pope John Paul I. The ordinal for King Juan Carlos I of Spain is used in both Spanish and English, though the British tradition of not using "I" as an ordinal until there is a "II" is sufficiently strong that he is sometimes simply called King Juan Carlos of Spain in English.
In Russia, this use of "The First" ordinal started with Paul I. Before him, neither Anna of Russia nor Elizabeth of Russia had the "I" ordinal.
There are cases when the national traditions were not respected. For example, when Mary I of England acceeded to the throne, her regnal style was announced as "the most high, most puissant, and most excellent Princess Mary the First, by the Grace of God Queen of England, France, and Ireland, Defender of the Faith, and of the Church of England and Ireland Supreme Head".
Between 1603, when the crowns were united in the person of James VI, and the union in 1707, the monarchs were numbered separately. After that, a single ordinal was used throughout Great Britain, and this has always been consistent with the English sequence of sovereigns. Hence, Edward VII of the United Kingdom was called Edward VII throughout the entire United Kingdom, even though he was only the first of that name to reign in Scotland.
In order to avoid controversy, it was announced after the accession of Elizabeth II that, in the future, the highest numeral from each sequence would be used. So any future British King Edward would be given the number IX, even though there have only been three previous Edwards in Scotland, but any future King Robert would be given the number IV, even though he would be the first Robert to reign in England.
Residents of Scotland were seemingly unaware of this convention upon the accession of the present Monarch in 1952, and made their feelings known. Objections were raised, and sustained, to the use of the Royal Cipher EiiR anywhere in Scotland, resulting in several violent incidents, including the destruction of one of the first new EiiR pillar boxes in Scotland, at Leith in late 1952. Since that time, the cipher used in Scotland on all Government and Crown property and street furniture has carried no lettering, but simply the Crown of Scotland from the Honours of Scotland.
Non-consecutive ordinals may indicate dynastic claims for non-regnant monarchs. For example, legitimists believed that, after Louis XVI of France was executed during the French Revolution, he was succeeded by his young son, who they called Louis XVII. Although the child died in prison a few years later and never reigned, his uncle, coming to the French throne after the revolution, took the name Louis XVIII in acknowledgement of his dynasty's rights. Similarly, when 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 distant cousin Louis Napoleon Bonaparte proclaimed himself Emperor in 1852, he declared himself Napoleon III of France in recognition of his predecessor.
The lack of an ordinal in the case of queens consort and princesses consort 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 after their deaths are recorded in history books and encyclopaedias through the use of their maiden name or, if from a noble family, its Royal House (dynastic name) or the name of the corresponding titled land.
Given the fact that she is only relatively recently deceased, people still refer to the widow of George VI by her last title, Queen Elizabeth the Queen Mother. However, she is increasingly being referred to in reference books and sources such as Wikipedia, and in time she may generally come to be referred to, as Elizabeth Bowes-Lyon. Similarly, though she is still widely referred to by her last title Diana, Princess of Wales, the late first wife of Charles, Prince of Wales is increasingly being referred to in sourcebooks, and may eventually be generally known as, Diana Spencer.