Primogeniture is when the oldest son inherits all or more of his parents' stuff than any of his siblings. When a king dies, typically by the rules of primogeniture, his eldest son inherits the throne.
It may seem vastly unfair, but primogeniture dates back to the Old Testament. Examples of this practice in which the first-born son receives exclusive inheritance rights can be found throughout history and around the world — from the Middle East to Medieval Europe. Today, perhaps the most widely known example of primogeniture is the succession of the British throne, which gives preference to the eldest male child above all others.