A viscountess should be addressed as "Lady C." by the upper classes, and as "My Lady" or "Your ladyship" by all other classes.
A baron should be addressed as "Lord D." by the upper classes, and as "My Lord" or "Your Lordship" by all other classes.
A baroness should be addressed as "Lady D." by the upper classes, and as "My Lady" or "Your Ladyship" by all other classes.
In strictly official or business intercourse a marquess, an earl, a viscount, a baron, and a younger son of a duke or marquis, should be addressed as "My Lord."
The eldest son of a duke should be addressed as "Lord A." by the upper classes, and as "My Lord" or "Your Lordship" by all other classes.
The wife of the eldest son of a duke should be addressed as "Lady A." by the upper classes, and as "My Lady" or "Your Ladyship" by all other classes.
The younger sons of a duke should be addressed as "Lord John E." or "Lord Charles E." by the upper classes, and as "My Lord" or "Your Lordship" by all other classes. Persons well acquainted with them would address them colloquially by their title and christian name, as "Lord John" or "Lord Charles." The same remark applies to their wives, who are often colloquially addressed as "Lady Alfred" or "Lady Edward."
The wives of the younger sons of a duke should be addressed as "Lady John E." or "Lady Charles E." by the upper classes, and as "My Lady" or "Your Ladyship" by all other classes.
The daughters of a duke should be addressed as "Lady Mary A." or "Lady Elizabeth B." by the upper classes, and as "Lady Mary" and "Lady Elizabeth" by those intimate with them, and as "My Lady" or "Your Ladyship" by all other classes.
The eldest son of a marquess should be addressed as "Lord A." by the upper classes, and as "My Lord" or "Your Lordship" by all other classes.