How did cadet branches start?

Question

In what ways could a cadet branch come into life? Why would a younger sibling in a royal or noble house start his own as a cadet branch?

This information is for personal, non-historical, writing; therefore the most useful answers would be ones that simply give me the reasons why it would happen in general. For instance, the specific political reasonings behind the making of a cadet branch in a single house I fear would simply go over my head. I would like the general reasons why it could happen that could apply to any royal or noble house, real or otherwise.

Answer 1

Appanage

Cadet branches arise under the system of Primogeniture when a younger son receives appanage and manages to establish it as a power base to his own line. Note that the "line" here is not an independent royal line, it is a line of (senior) vassals to the main royal line. If the main line dies out, the cadet line will claim the throne, but otherwise it will merely supply spouses and (usually senior) royal officials.

It is possible that a cadet branch will eventually split its fief from the main kingdom (if the central government grows weak), in which case it will no longer be referred to as a cadet line, but an independent "house of so-and-so". However, it is more likely that in such a case it will exploit the main line's weakness and its own lineage to seize the throne.

Career

When younger sons are instead (of granting them appanage) encouraged to pursue a career (either military or religious), no cadet branch is usually established, as they either have no descendants due to celibacy or death on the battlefield or marry into another royal (or just noble) house.

Answer 2

As another example, I would refer to the Hauteville family in Normandy. There were several physically fit sons, and to limit the division of the already modest estates, the younger sons were sent off to conquer their own domains among the Italian states. They did so successfully, under their leader Robert, who then established the Guiscard clan, and a descendant was acknowledged by the Pope as King of Sicily.

Answer 3

Typically, nobles would leave their estates to their oldest sons in order to transfer them intact within a family line, and to SUPPRESS "cadet" branches. One major exception was when a younger son was unusually meritorious, particularly in battle, to the point of putting his older brothers to shame.

One example was when Philip the Bold, the youngest son of King John of France, accompanied his father into the Battle of Poitiers during the 100 Years' War (none of his brothers did this). John rewarded Philip with the Duchy of Burgundy.

The end result represented precisely the reason why nobles tried to avoid "cadet" branches. Philip married the Countess of Flanders, which was hostile to France, and two generations later, Burgundy-Flanders allied with England against France, after the battle of Agincourt.

Another example of a "cadet branch" was the Duke of Wellington. He was the third son of a count, but he was "promoted" to Duke (two levels higher than his father), because of his success in battle, notably at Waterloo.