All crowned heads visiting England should be addressed as "Sir" by those socially known to them, and as "Your Majesty" by all others. The Royal ladies, their wives, should be addressed as "Ma'm" by those personally known to them, and as "Your Majesty" by all others.
A foreign prince bearing the title of Serene Highness should be addressed as "Prince," and not as "Sir," by the aristocracy and gentry, and as "Your Serene Highness" by all other classes.
A foreign princess, also bearing the title of serene highness, should be styled "Princess" when addressed colloquially by the upper classes, but not as "Ma'am"; and as "Your Serene Highness" by all other classes.
An English duke should be addressed as "Duke" by the aristocracy and gentry, and not as "Your Grace" by members of either of these classes. All other classes should address him colloquially as "Your Grace."
An English duchess should be addressed as "Duchess" by all persons conversing with her belonging to the upper classes, and as "Your Grace" by all other classes.
A marquess, colloquially, should be addressed as "Lord A."
A marchioness should be addressed as "Lady A." by the upper classes. It would be a mistake to address an English marquess as "Marquess," or a marchioness as "Marchioness," colloquially speaking. All other classes should address them either as "My Lord" or "Your Lordship," "My Lady" or "Your Ladyship."
An earl should be addressed as "Lord B." by the upper classes, and as "My Lord" or "Your Lordship" by all other classes.
A countess should be addressed as "Lady B." by the upper classes, and as "My Lady" or "Your Ladyship" by all other classes.
A viscount should be addressed as "Lord C." by the upper classes, and as "My Lord" or "Your Lordship" by all other classes.