An order, line or right of succession is the line of individuals necessitated to hold a high office when it becomes vacated, such as a head of state or an honour such as a title of nobility.UK Royal Web site "The order of succession is the sequence of members of the Royal Family in the order in which they stand in line to the throne. This sequence is regulated not only through descent, but also by Parliamentary statute." This sequence may be regulated through descent or by statute.

Hereditary government form differs from elected government. An established order of succession is the normal way of passing on hereditary positions, and also provides immediate continuity after an unexpected vacancy in cases where office-holders are chosen by election: the office does not have to remain vacant until a successor is elected. In some cases the successor takes up the full role of the previous office-holder, as in the case of the presidency of many countries; in other non-hereditary cases there is not a full succession, but a caretaker chosen by succession criteria assumes some or all of the responsibilities, but not the formal office, of the position. For example, when the position of President of India becomes vacant, the Vice President of India temporarily carries out the functions of the presidency until a successor is elected; in contrast, when the position of President of the Philippines is vacant, the Vice President of the Philippines outright assumes the presidency itself for the rest of the term.

Organizations without hereditary or statutory order of succession require succession planning if power struggles prompted by power vacuums are to be avoided.

Research shows that authoritarian regimes that relied on primogeniture for succession were more stable than forms of authoritarian rule with alternative succession arrangements. Scholars have linked primogeniture to a decline in regicide, as clear rules of succession reduce the number of people who could (absent a coup d'état) replace a ruler, thus making it less desirable to cause the death of the monarch.

Overview It is often the case that the inheritance of a hereditary title, office or the like, is indivisible: when the previous holder ceases to hold the title, it is inherited by a single individual. Many titles and offices are not hereditary (such as democratic state offices) and they are subject to different rules of succession.

A hereditary line of succession may be limited to heirs of the body, or may also pass to collateral lines, if there are no heirs of the body, depending on the succession rules. These concepts are in use in English inheritance law.

The rules may stipulate that eligible heirs are heirs male or heirs general – see further primogeniture (agnatic, cognatic, and also equal).

Certain types of property pass to a descendant or relative of the original holder, recipient or grantee according to a fixed order of kinship. Upon the death of the grantee, a designated inheritance such as a peerage, or a monarchy, passes automatically to that living, legitimate, non-adoptive relative of the grantee who is most senior in descent (i.e. highest in the line of succession, regardless of age); and thereafter continues to pass to subsequent successors of the grantee, according to the same rules, upon the death of each subsequent heir.

Each person who inherits according to these rules is considered an heir at law of the grantee and the inheritance may not pass to someone who is not a natural, lawful descendant or relative of the grantee.

Collateral relatives, who share some or all of the grantee's ancestry, but do not directly descend from the grantee, may inherit if there is no limitation to "heirs of the body".

There are other kinds of inheritance rules if the heritage can be divided: heirs portioners and partible inheritance.

Monarchies and nobility In hereditary monarchies the order of succession determines who becomes the new monarch when the incumbent sovereign dies or otherwise vacates the throne. Such orders of succession, derived from rules established by law or tradition, usually specify an order of seniority, which is applied to indicate which relative of the previous monarch, or other person, has the strongest claim to assume the throne when the vacancy occurs.

Often, the line of succession is restricted to persons of the blood royal (but see morganatic marriage), that is, to those legally recognized as born into or descended from the reigning dynasty or a previous sovereign. The persons in line to succeed to the throne are called "dynasts". Constitutions, statutes, house laws, and norms may regulate the sequence and eligibility of potential successors to the throne.

Historically, the order of succession was sometimes superseded or reinforced by the coronation of a selected heir as co-monarch during the life of the reigning monarch. Examples are Henry the Young King and the heirs of elective monarchies, such as the use of the title King of the Romans for the Habsburg emperors. In the partially elective system of tanistry, the heir or tanist was elected from the qualified males of the royal family. Different monarchies use different rules to determine the line of succession.

Hereditary monarchies have used a variety of methods and algorithms to derive the order of succession among possible candidates related by blood or marriage. An advantage of employing such rules is that dynasts may, from early youth, receive grooming, education, protection, resources and retainers suitable for the future dignity and responsibilities associated with the crown of a particular nation or people. Such systems may also enhance political stability by establishing clear, public expectations about the sequence of rulers, potentially reducing competition and channeling cadets into other roles or endeavors.

Some hereditary monarchies have had unique selection processes, particularly upon the accession of a new dynasty. Imperial France established male primogeniture within the descent of Napoleon I, but failing male issue the constitution allowed the emperors to choose who among their brothers or nephews would follow them upon the throne. The Kingdom of Italy was designated a secundogeniture for the second surviving son of Napoleon I Bonaparte but, failing such, provided for the emperor's stepson, Eugène de Beauharnais, to succeed, even though the latter had no blood relationship to the House of Bonaparte. Serbia's monarchy was hereditary by primogeniture for male descendants in the male line of Prince Alexander I, but upon extinction of that line, the reigning king could choose any among his male relatives of the House of Karađorđević. In Romania, on the other hand, upon extinction of the male line descended from Carol I of Romania, the constitution stipulated that the male line of his brother, Leopold, Prince of Hohenzollern, would inherit the throne and, failing other male line issue of that family, a prince of a "Western European" dynasty was to be chosen by the Romanian king and parliament. By contrast, older European monarchies tended to rely upon succession criteria that only called to the throne descendants of past monarchs according to fixed rules rooted in one or another pattern of laws or traditions.

Vertical inheritance In hereditary succession, the heir is automatically determined by pre-defined rules and principles. It can be further subdivided into horizontal and vertical methods, the former favoring siblings, whereas vertical favors children and grandchildren of the holder.

Male-preference cognatic primogeniture

In male-preference primogeniture (in the past called cognatic primogeniture) the monarch's eldest son and his descendants take precedence over his siblings and their descendants. Elder sons take precedence over younger sons, but all sons take precedence over daughters. A female member of a dynasty can succeed to the throne if and only if she has no living brothers and no deceased brothers who left surviving legitimate descendants. Older daughters and their descendants take precedence over younger daughters and their descendants. Children represent their deceased ancestors, and the senior line of descent always takes precedence over the junior line, within each gender. The right of succession belongs to the eldest son of the reigning sovereign (see heir apparent), and next to the eldest son of the eldest son.

This is the system in Spain and Monaco, and was the system used in the ancient kingdoms of Portugal, of England and Scotland and later, the Kingdom of Great Britain and finally, the United Kingdom and the Commonwealth realms (although, during the Middle Ages, Scotland also supported tanistry and proximity of blood, through both male and female lines, due to the intermingling of Pictish and Gaelic succession rules).

With respect to hereditary titles, it is usually the rule everywhere in Scotland and baronies by writ in the United Kingdom, but usually these English baronies by writ go into abeyance when the last male titleholder dies leaving more than one surviving sister or more than one descendant in the legitimate female line of the original titleholder. In England, Fiefs or titles granted "in tail general" or to "heirs general" allow older sons to succeed before younger sons, but daughters are considered equal co-heirs to each other, which can result in abeyance. In the medieval period, actual practice varied with local custom. While women could inherit manors, power was usually exercised by their husbands (jure uxoris) or their sons (jure matris). However, in Scotland, Salic law or any of its variations have never been practised, nor abeyance, and all the hereditary titles are inherited through male-preference primogeniture, where in the extinction of a male line, the eldest sister automatically receives the titles, and rules in her own right, not in the right of her son. A famous example of this is Marjorie, Countess of Carrick, mother of Robert the Bruce, who was the Countess of Carrick in her own right.

A similar system was practised in many of the kingdoms of the Indian subcontinent from the Middle Ages to the Indian independence movement. In many of these kingdoms, adoption was allowed from a relative if a monarch did not have children, and the adopted child could succeed to the throne at the death of the monarch. Examples include Rajaram II of Satara, Shahu II of Satara and Princess Bharani Thirunal Parvathy Bayi, the mother of the reigning Queen Gowri Lakshmi Bayi of Travancore. Often, the wife or mother of a childless king succeeded to the throne and ruled in her own right until her death, after which the throne passed to the next closest relative. Prominent examples of this custom include Queen Didda of Kashmir, and Qudsia Begum, the Nawab of Bhopal. Razia Sultana of the Delhi Sultanate was a rare example of a queen who succeeded her father even when her brothers were alive.

Absolute cognatic primogeniture

Absolute primogeniture is a law in which the eldest child of the sovereign succeeds to the throne, regardless of gender, and females (and their descendants) enjoy the same right of succession as males. This is currently the system in Sweden (since 1980), the Netherlands (since 1983), Norway (since 1990), Belgium (since 1991), Denmark (since 2009), Luxembourg (since 2011), and in the United Kingdom and the Commonwealth realms (since 2013; only for those born after 28 October 2011).Explanatory note to the UK bill , paragraph 42: "There is power to specify the time of day of commencement. Assuming that the other Realms make the same provision, this will enable the changes on succession to be brought into force at the same time – but at different local times – in all sixteen Commonwealth Realms." UK parliament official website. (Retrieved 30 March 2015.)

Agnatic succession (Salic law)