The political system of ancient Aurlia valued large, strong families and dynasties. This kept the power stable, and allowed for a smooth transition of power should the King perish.
Upon the King's death, the crown would go to his eldest legitimate son. Should the King have no sons, the crown would go to his eldest legitimate daughter. If the King should perish before his heir has come of age, the Regent would rule as the King. Ideally, this would teach the new King how to be a King. Though, there have been many cases where the Regent would seize the throne for himself.
Of course, the King had the right to select their heir. However, deviations from the norm could put strain in the King's key supporters, and so it was in the King's best interest to discuss these decisions with their court.
The ruling family could only remain in power should the court allow them. A position in the royal court is still a position of power, and if any of them start to get worried, they could replace the King at any time. A good King would know this, and would work to keep his key supporters happy. It was especially important for the King to keep his Blade and Regent happy, as their word carried the same weight as the King's. Should they deem it necessary, the Blade and Regent could simply allow a coup to occur, replacing the King with one someone they chose. Most successful coups in Aurlia only happened because the Blade and Regent allowed it to happen.
The role of the King's spouse changed between cultures and the current ruling family. The King's spouse was usually not granted any special rights beyond being related to the King. The King's spouse could also have been seen as a cultural representative, another Regent, a diplomat, or as co-King. There have also been times when the King's spouse functionally acted as the King himself.
It was essential for the King to have many children. Both male and female children were deemed necessary: having many sons meant having many potential heirs, their daughters could be offered to other families in order to establish alliances.
This system was established by humans, and spread across Aurlia through the Callinian Empire. Smaller settlements and cultures often had to mimic this structure when interacting with larger powers, even if it is just temporarily. Establishing these temporary Kings was a particular enjoyment of the halflings.
