Head of state
By hereditary succession

The position of a monarch is usually hereditary, but in constitutional monarchies, there are usually restrictions on the incumbent's exercise of powers and prohibitions on the possibility of choosing a successor by other means than by birth. In a hereditary monarchy, the position of monarch is inherited according to a statutory or customary order of succession, usually within one royal family tracing its origin through a historical dynasty or bloodline. This usually means that the heir to the throne is known well in advance of becoming monarch to ensure a smooth succession. However, many cases of uncertain succession in European history have often led to wars of succession.

Primogeniture, in which the eldest child of the monarch is first in line to become monarch, is the most common system in hereditary monarchy. The order of succession is usually affected by rules on gender. Historically "agnatic primogeniture" or "patrilineal primogeniture" was favoured, that is inheritance according to seniority of birth among the sons of a monarch or head of family, with sons and their male issue inheriting before brothers and their male issue, to the total exclusion of females and descendants through females from succession. This complete exclusion of females from dynastic succession is commonly referred to as application of the Salic law. Another variation on agnatic primogeniture was the so-called semi-Salic law, or "agnatic-cognatic primogeniture", which allowed women to succeed only at the extinction of all the male descendants in the male line of the particular legislator.

Before primogeniture was enshrined in European law and tradition, kings would often secure the succession by having their successor (usually their eldest son) crowned during their own lifetime, so for a time there would be two kings in coregency – a senior king and a junior king. Examples include Henry the Young King of England and the early Direct Capetians in France.

Sometimes, however, primogeniture can operate through the female line. In some systems a female may rule as monarch only when the male line dating back to a common ancestor is exhausted. In 1980, Sweden, by rewriting its 1810 Act of Succession, became the first monarchy to declare equal (full cognatic) primogeniture, meaning that the eldest child of the monarch, whether female or male, ascends to the throne. Other European monarchies (such as the Netherlands in 1983, Norway in 1990 and Belgium in 1991) have since followed suit. Similar reforms were proposed in 2011 for the United Kingdom and the other Commonwealth realms, which came into effect in 2015 after having been approved by all of the affected nations. Sometimes religion is affected; under the Act of Settlement 1701 all Roman Catholics and all persons who have married Roman Catholics are ineligible to be the British monarch and are skipped in the order of succession.

In some monarchies there may be liberty for the incumbent, or some body convening after the death of the monarch, to choose from eligible members of the ruling house, often limited to legitimate descendants of the dynasty's founder. Rules of succession may be further limited by state religion, residency, equal marriage or even permission from the legislature.

Other hereditary systems of succession included tanistry, which is semi-elective and gives weight to merit and Agnatic seniority. In some monarchies, such as Saudi Arabia, succession to the throne usually first passes to the monarch's next eldest brother, and only after that to the monarch's children (agnatic seniority).
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