If the heir to a Kingdom is called Crown Prince, what is the heir to a Duchy called?

Question

Just like the title said. What a heir to a hereditary Duchy is called? (as opposed to someone appointed by a King). I mean a Duchy where the Duke inherited his title to his son/daughter. Since the only sovereign Duchy in the world is Grand Duchy of Luxembourg where they styled the heir as "Hereditary Grand Duke", then is by the same analogy, the heir of a hereditary Duchy is "Hereditary Duke"?

Answer 1

Normally, heirs to duchies were given the title "Hereditary Prince" and were referred to as Prince XXXX. An example of this would be the duchy of Modena (Italy), where the heir to the Duke of Modena was Hereditary Prince of Modena.