How do you address a letter to a Sir and a Lady together?

When addressing a letter to a Sir and a Lady together, you should use the format "Sir [First Name] [Last Name] and Lady [First Name] [Last Name]." For example, if addressing Sir John Smith and Lady Jane Smith, you would write: "Sir John Smith and Lady Jane Smith." In the salutation, you can start with "Dear Sir John and Lady Jane," or simply "Dear Sir and Lady Smith."