Thomas Davidson

“ Duke—'His Grace the Duke of ——.' Begin: 'My Lord Duke.' Refer to as 'Your Grace.'
Duke's Daughter—'The Right Hon. Lady' [Christian name and surname] , or 'The Lady' [Christian name and surname] , the surname being that of her husband if married. Begin: 'Madam.' Refer to as 'Your Ladyship.' If married to a peer, she is addressed according to her husband's rank only. This, however, does not hold in the case of peers by courtesy; and a Duke's daughter married to the eldest son of an Earl, after the prefix 'Lady,' sometimes takes her own Christian name, followed by her husband's courtesy title. ”

“ Persons holding offices other than those enumerated are addressed in the usual form, 'Sir,' 'Dear Sir,' or 'My dear Sir,' according to the more or less formal terms on which the writer may be with his correspondent. ”