Question: how to address hrh in a letter?
Answer: If communicating directly with a member of the Royal Family the letter should begin 'Sir/Madam' and end 'I have the honour to remain, Sir/Madam, Your Royal Highness's most humble and obedient servant'. In the body of the letter substitute 'Your Royal Highness' for 'you' and 'Your Royal Highness's' for 'your'.