By the 17th century, all local rulers in Italy adopted the style
Highness, that was once used by kings and emperors only. According to
Encyclopédie, the style of Royal Highness was created on the insistence of Archduke
Ferdinand of Austria, Cardinal-Infante of Spain, a younger son of King
Philip III of Spain. The Archduke was travelling through Italy on his way to the Low Countries and, upon meeting
Victor Amadeus I, Duke of Savoy, refused to address him as Highness unless the Duke addressed him as Royal Highness. Thus, the first use of the style Royal Highness was recorded in 1633.
Gaston, Duke of Orléans, younger son of King
Henry IV of France, encountered the style in
Brussels and assumed it himself. His children later used the style, considering it their prerogative as
grandchildren of France.
By the 18th century, Royal Highness had become the prevalent style for members of a
continental reigning dynasty whose head bore the hereditary title of king or queen. The titles of family members of non-hereditary rulers (e.g., the
Holy Roman Emperor,
King of Poland,
Princes of Moldavia and Wallachia—and even the kin of the
Princes of Orange who held hereditary leadership though not monarchical position in much of the Netherlands, etc.) were less clear, varying until rendered moot in the 19th century. After dissolution of the Holy Roman Empire in 1806, several of Germany's
prince-electors and other now sovereign rulers assumed the title of
grand duke and with it, for themselves, their eldest sons and consorts, the style of Royal Highness (Baden, Hesse, Mecklenburg, Saxe-Weimar).