Prince of the blood

A prince du sang (French pronunciation: ​[pʁɛ̃s dy sɑ̃], Prince of the Blood) is a person legitimately descended in dynastic line from any of a realm's hereditary monarchs. Historically, the term has been used to refer to men and women descended in the male line from a sovereign, although as absolute primogeniture has become more common in monarchies, those with succession rights through female descent are more likely than in the past to be accorded the princely title (e.g., Belgian Royal Family).

In some European kingdoms, especially France, this appellation was a specific rank in its own right, of a more restricted use than other titles.

Under the House of Capet, the monarchy was feudal, and the younger sons and grandsons of kings did not have rights or precedence based on their royal descent. Feudal titles determined rank. Under Philip Augustus, the Duke of Burgundy, a peer of France, could be reckoned to be mightier than the Count of Dreux, a "baron of the second rank", even though the latter is a paternal cousin of the king, while the former was only a distant agnate. In the feudal era, the agnates of the king held no special status. This was because agnatic primogeniture had not yet received its sanction as the law governing the succession to the French throne.

Following the Valois succession, the agnates of the king, being "capable of the crown", rose in prominence. New peerages were created for the king's agnates, and for a long time this continued to be so, before the peerage was extended to non-royalty. Over time, the dignity of a peer, which is feudal in nature, and the dignity of a prince of the blood, which is dynastic in nature, clashed. Non-royal peers and princes of the blood who were peers constantly disputed for higher precedence than the other. As the royal line contracted, each prince of the blood gained greater prominence. Finally, in 1576, Henry III of France issued an edict, as a counterpoise to the growing power of the House of Guise, which made the princes of the blood supreme over the peerage, and amongst themselves, the closer in the line of succession would outrank the more distant, without regard to the actual title that they held.

...
Wikipedia