A Duchess is addressed as 'Her Grace' as a formal honorific title. Similarly, a Baronet is addressed as 'Sir'. Both pairs show the relationship between a noble title and their form of address. Monsignors use 'Your Reverence' or 'Monsignor', Earls use 'The Right Honourable' (not 'The Honourable'), and Nuns use 'Sister' (not 'Mother').