Fils de France

Fils de France (French pronunciation: ​[fis də fʁɑ̃s], Son of France) was the style and rank held by the sons of the kings and dauphins of France. A daughter was known as a fille de France (French pronunciation: ​[fij də fʁɑ̃s], Daughter of France).

The children of the dauphin (a title reserved for the king's heir apparent whether son, grandson or great-grandson of the monarch) were accorded the same style and status as if they were the king's children instead of his grandchildren or great-grandchildren.

The king, queen, queen dowager, enfants de France (Children of France) and petits-enfants de France (Grandchildren of France) constituted the famille du roi (Royal Family). More remote legitimate, male-line descendants of France's kings held the designation and rank of princes du sang (Princes of the Blood) or, if legally recognised despite a bar sinister on the escutcheon, they were customarily deemed princes légitimés (Legitimated Princes).

The dauphin, the heir to the French throne, was the most senior of the fils de France and was usually addressed as Monsieur le dauphin. The king's next younger brother, also a fils de France, was known simply as Monsieur, and his wife as Madame.

Daughters were referred to by their given name prefaced with the honorific Madame, while sons were referred to by their main peerage title (usually ducal), with the exception of the dauphin. The king's eldest daughter was known as Madame Royale until she married, whereupon the next eldest fille de France succeeded to that style.

...
Wikipedia