Her Highness殿下(女性王室成员的间接指称)

Her Royal Highness the Duchess of Kent opened an art exhibition recently.肯特公爵夫人殿下最近为一次画展揭幕。

Your Highness殿下, 阁下(用于对王室成员的尊称)

That would be best, Your Highness.那样就再好不过了, 殿下。