A prince could theoretically have been born the last of (say) five sons in his family, but still succeeded to the throne because his older brothers all died in infancy. But he would still count as a youngest son who became a reigning monarch.
King Alfonso XIII of Spain, however, would not -- for reasons that he was not preceded in birth by a brother who died in infancy. Holy Roman Emperor Ferdinand I (founder of the Austrian Hapsburgs), however, does count -- despite having only one older brother (Holy Roman Emperor Charles V, founder of the Spanish Habsburgs).
Later on in the history of the Habsburgs, Archduke Ferdinand Karl (youngest son of Holy Roman Emperor Francis I) became the Duke of Bresgau and founder of the cadet branch known as Austria-Este.
Prince Valdemar of Denmark (youngest son of King Christian IX) would have made the list, but for the fact that he refused the throne of Bulgaria which got offered to him.
King John of England, however, counts -- despite having usurped the throne. There was no clear succession law as of his day, where basically possession was nine-tenths of the law. Similarly, King Henry I also counts, for reasons that he was the youngest son of William of Normandy ("the Conqueror").
Can anybody name additional examples?
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