Monarchy
Hereditary monarchies

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.

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 were Henry the Young King of England and the early Direct Capetians in France. Sometimes, however, primogeniture can operate through the female line.

In 1980, Sweden 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 kingdoms (such as the Netherlands in 1983, Norway in 1990, Belgium in 1991, Denmark in 2009, and Luxembourg in 2011) have since followed suit. The United Kingdom adopted absolute (equal) primogeniture (subject to the claims of existing heirs) on April 25, 2013, following agreement by the prime ministers of the sixteen Commonwealth Realms at the 22nd Commonwealth Heads of Government Meeting.

In the absence of children, the next most senior member of the collateral line (for example, a younger sibling of the previous monarch) becomes monarch. In complex cases, this can mean that there are closer blood relatives to the deceased monarch than the next in line according to primogeniture. This has often led, especially in Europe in the Middle Ages, to conflict between the principle of primogeniture and the principle of proximity of blood.

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 first passes to the monarch's next eldest brother, and only after that to the monarch's children (agnatic seniority). However, on June 21, 2017, King Salman of Saudi Arabi revolted against this style of monarchy and elected his son to inherit the throne.
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