Grand Ducal Highness

His or Her Grand Ducal Highness (abbreviation: HGDH) is a style of address used by the non-reigning members of some German ruling families headed by a Grand Duke. No currently reigning family employs the style, although it was used most recently by the younger sisters of the late Grand Duchess Charlotte of Luxembourg. Since Grand Duchess Charlotte's marriage to Prince Felix of Bourbon-Parma, all of their male-line descendants have used the style Royal Highness, which he bore.

A reigning grand duke, his heir apparent, and their spouses would use the style of Royal Highness. The male line descendants of a reigning grand duke, other than the heir, would use the style Grand Ducal Highness. This practice was followed by the ruling families of Luxembourg, Hesse and by Rhine, and Baden. Other grand ducal families had either ceased to reign as grand dukes by the time this system developed following the 1815 Congress of Vienna or accorded only the style of "Highness" to cadet (Mecklenburg-Schwerin, Mecklenburg-Strelitz, Oldenburg, Saxe-Weimar). At present, the style is used only by the former ruling family of Baden, as the Hessian grand ducal family has become extinct.

Russian grand dukes and grand duchesses were the children or grandchildren of an Emperor of Russia and used the style Imperial Highness. The Grand Dukes of Tuscany used the style Royal Highness for themselves but it is not clear what style other members of the family would have used. By the time the system of different classes of Highness came into regular use for the relatives of rulers (in the eighteenth century), the Tuscan grand dukes were also members of the House of Austria-Hungary. As such, they had the title of Archduke and used the style Imperial and Royal Highness.

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