The title of "Grand Ducal Highness" is a style within the Pontifical Imperial State of Rome-Ruthenia and the United Roman-Ruthenian Church used by the junior members of the families of Grand Dukes (holding a titular Grand Duchy, distinct from the Grand Dukes/Duchesses of the Pontifical and Imperial Household).
This title is also used by members of certain Grand Ducal Houses of the Holy Roman Empire.