In 1759, King Philip's younger grandson was appanaged with the kingdoms of Naples and Sicily, becoming Ferdinand IV and III (1751–1825), respectively, of those realms.

The crowns of Naples and Sicily remained functionally separate, albeit often ruled by the same monarch, until their formal union in 1816.

When Prince Ferdinand Pius died in 1960, he left no male descendant, and two branches of the family claimed the right to succeed him as head of the house.

Despite an investigation by five of the highest institutions of the Spanish state having concluded unanimously that the legitimate claimant was the late Infante Carlos, Duke of Calabria, the junior line continues to perpetuate its claim.

The rule of absolute primogeniture would henceforth apply to his direct descendants, his elder daughter being named by him as heiress apparent.

Other agnatic descendants of the King, born of authorized marriages, bore the title Prince(ss) of the Two Sicilies with the style of Royal Highness.