Younger sons founded cadet branches, and some of those branches ended up on thrones
Under primogeniture, the eldest son took the crown, the lands and the income, while younger sons got scraps. Their descendants formed cadet branches, freer to go into trade, the army or the colonies. Yet again and again those junior lines outlasted the senior ones and seized or inherited the top job.
Primogeniture developed to stop estates being split into ever-smaller pieces. Where holdings were divided equally among sons, as often happened in the Holy Roman Empire, inheritances could shrink until they were too small to support, manage or defend, and brothers fought over shares. Handing almost everything to the firstborn kept family power intact, at the younger sons' expense. A cadet was not even expected to marry, but if he did, his line could take on work seen as too risky or demeaning for the heir: law, the church, the military, commerce or emigration.
The military route made at least one famous name. Arthur Wellesley, a younger brother of the Earl of Mornington, entered the army as younger sons traditionally did, and his victories between 1809 and 1814 carried him through a string of titles to Duke of Wellington, far outranking his brother's earldom.
Many royal houses began as cadets. The Bourbons, descended from a younger son of Louis IX, took the French throne from the Valois in 1589, and branches of the family later reigned in Spain, Naples and Luxembourg. The House of York was a cadet line of the Plantagenets that displaced the Lancastrians during the Wars of the Roses. A small branch of the Wettin family ruling Saxe-Coburg and Gotha, through shrewd marriages, supplied monarchs to Belgium, Portugal, Bulgaria and Britain.
Being junior could also mean being passed over. Nelson Mandela descended in the male line from a Thembu king, but because the branch stemmed from a marriage to a woman of lower ritual rank, his family could not inherit and served instead as hereditary royal counsellors. In Saudi Arabia, cadet branches of the Al Saud hold powerful posts but stand outside the line of succession.
Source: Cadet branch
