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.
The right of succession belongs to the eldest son of the reigning sovereign (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 Commonwealth realms for those born before 2011.
Absolute cognatic primogeniture
the eldest child of the sovereign succeeds to the throne, regardless of gender, and females
(or semi-Salic) succession.
is the restriction of succession to those descended from or related to a past or current monarch exclusively through the male line of descent
In this form of succession, the succession is reserved first to all the male dynastic descendants of all the eligible branches by order of primogeniture,
then upon total extinction of these male descendants to a female member of the dynasty.[The only current monarchy that operated under semi-Salic law until recently is Luxembourg, which changed to absolute primogeniture in 2011.
restricted the pool of potential heirs to males of the patrilineage
The Salic law applied to the former royal or imperial houses of Albania, France, Italy, Romania, Yugoslavia, and Prussia/German Empire. It currently applies to the house of Liechtenstein, and the Chrysanthemum Throne of Japan.
In 1830 in Spain the question whether or not the Salic law applied – and therefore, whether Ferdinand VII should be followed by his daughter Isabella or by his brother Charles – led to a series of civil wars and the formation of a pretender rival dynasty which still exists.