What is primogeniture?

Primogeniture is a rule of inheritance whereby the eldest child, and normally the eldest male child, is the heir.
It might be applied to property and/or titles. Historically, primogeniture was normal for titles in England, Scotland, Ireland, France, Spain and southern Italy for titles following Frankish’ law in southern Italy and Sicily. Since 2006, Spanish titles have been inheritable by the eldest child regardless of sex.