Question: what is the difference between your grace and my lord?
Answer: Your Grace is, in modern English, the correct way to speak to either a Duke or a Bishop. Wheras “My Lord” can refer to a Marquess, Earl, Viscount or Baron, and “My Lords” to refer to a mixed group of them, or a group that includes them and Dukes. (A group of Dukes is “your graces”.)