Primogeniture is the right of a child to the throne. At this moment there are 3 types of primogeniture used by royal and princely houses at the world.
Absolute, equal(full) cognatic or lineal primogeniture is a form of primogeniture in which sex is irrelevant for inheritance. The oldest surviving child without regard to sex inherits the throne. It is often used after the second part of the XX century. At this moment this type is using in Sweden, United Kingdom, Netherlands, Norway, Belgium, Denmark, Luxemburg, and Nepal before the monarchy was abolished. Most of Princely and Ducal houses are using this type of succession nowadays.
Male primogeniture gives the right to the throne exclusively to the male descendant by the,, first breath”, to the firstborn son and is used only in Monaco and Spain. Older sons with their sons come before younger sons and their lines.
Agnatic-cognatic primogeniture allows women to succeed only in case there is no male descendants. At this moment this type is used in one African state, one state in Latin America, and two states in the Middle East.
Princely and Ducal House is based on Absolute, equal primogeniture.