The colloquial application of titles differs materially from the application of titles when not used colloquially, and many persons are in doubt as to whether they should or should not make use colloquially of titles in full.

His Majesty the King should be addressed as “Sir” by all those who come in social contact with him; and by all others as “Your Majesty.”

Her Majesty Queen Mary should be addressed as “Ma’m” by all those who come in social contact with her; and by all others as “Your Majesty.”

Her Majesty Queen Alexandra should be addressed as “Ma’m” by all those who come in social contact with her; and by all others as “Your Majesty.”

The Prince of Wales, the Duke of Connaught, and all princes of the blood royal, should be addressed by the upper classes as “Sir.”

The princesses of the blood royal, should be addressed as “Ma’m” by the upper classes. The wives of the princes of the blood royal should also be addressed as “Ma’m” by the upper classes.

The German Emperor (Wilhelm II, Queen Victoria’s grandson) should be addressed as “Sir” by the upper classes.

The King of Spain (Alfonso XIII) should be addressed as “Sir” by the upper classes; the Queen of Spain (Princess Victoria Eugenie of Battenberg, Queen Victoria’s granddaughter) should be addressed as “Ma’m” by the upper classes.

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.

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.

The wife of the eldest son of a marquis 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 marquis should be addressed as “Lord Henry B.” and “Lord Frederick B.” by the upper classes, and as “My Lord” or “Your Lordship” by all other classes.

The wives of the younger sons of a marquis should be addressed as “Lady Henry B.” and “Lady Frederick B.” by the upper classes, and as “My Lady” or “Your Ladyship” by all other classes.

The daughters of a marquis should be addressed as “Lady Florence B.” and “Lady Sarah B.” by the upper classes, and as “My Lady” or “Your Ladyship” by all other classes.

The eldest son of an earl should be addressed as “Lord C.” by the upper classes, and as “My Lord” or “Your Lordship ” by all other classes.

The wife of the eldest son of an earl should be addressed as “Lady C.” by the upper classes, and as “My Lady” or “Your Ladyship” by all other classes.

The daughters of an earl should be addressed as “Lady Blanche ” and “Lady Evelyn” by the upper classes, and as “My Lady” or “Your Ladyship” by all other classes.

The younger sons of earls, and both eldest and younger sons of viscounts and barons, only bear the courtesy title of honourable. The daughters of viscounts and barons also bear the courtesy title of honourable. But this title of honourable should never be used colloquially under any circumstances. The Honourable Mr. or Mrs. B., or the Honourable Miss B., should be styled Mr., Mrs., or Miss B.

Baronets should be addressed by their full title and surname, as Sir John Blank, by the upper classes, and by their titles and christian names only by all other classes.

Baronets’ wives should be addressed as “Lady B.” or “Lady C,” according to the surnames of their husbands: thus, “Sir John Blank’s” wife should be addressed as “Lady Blank” by the upper classes, not as “Lady John Blank”—to do so would be to give her the rank of the wife of the younger son of a duke or marquis instead of that of a baronet’s wife only—and as “My Lady” or “Your Ladyship” by all other classes.

The wives of knights should be addressed as “Lady B.” or ” Lady C,” according to the surnames of their husbands: thus, “Sir John Blank’s” wife should be addressed as “Lady Blank” by the upper classes, and as “My Lady” or “Your Ladyship” by all other classes.

Source: Manners and Rules of the Aristocracy (1913) by A Member of the Aristocracy