Prefixes to Titles
In official documents and on very formal occasions a duke is styled "The Most Noble the Duke of", but most commonly "His Grace the Duke of"; marquesses are styled "The Most Honourable the Marquess of", and earls, viscounts and barons "The Right Honourable". Peeresses take the style of their husbands.
I am often asked whether in addressing communications to peers these styles should be used. In the case of ordinary correspondence the answer is "No".
Communications to a peer from the College of Arms, the House of Lords or the Lord Chamberlain's Department merely prefix the definite article to his title. "The Duke of This", "The Viscount That", "The Lord Another". It may be noticed that the Court Circular, while it punctiliously refers to, say, "the Right Hon. Winston Spencer-Churchill" or "the Right Hon. Clement Attlee" when alluding to commoners who are Privy Counsellors, follows this style of using the definite article only in regard to peers. Similarly, it refers to "the Viscountess Hambleden" as being in attendance on the Queen.
The College of Arms, representing the Earl Marshal, I understand, holds that these prefixes, whether in full formal style or consisting of the definite article, should be applied only to actual peers and peeresses. But various Lord Chamberlains, when consulted on this point, have ruled that their use is in order not only for the eldest sons of peers who bear courtesy peerage titles, but also for all children or grandchildren of peers entitled to prefix the courtesy style of Lord or Lady to their names. This rule is followed by the Court Circular, an official publication, which you will find referring to "the Lady Herbert" (wife of a peer's eldest son) as being in attendance on the Duchess of Kent, and to "the Lady Constance Milnes-Gaskell" as attending Queen Mary.
But the Earl Marshal - who, it is claimed, is the higher authority in questions of dignity - holds otherwise, vide the Coronation ceremonial issued by him, in which "the" is omitted for all courtesy titles. The Court Circular, it is argued, may be an official publication but it is not the official publication, because those who compile it are not the ultimate authority. "The", maintains the College of Arms, indicates the person who holds the titles, just as you say "The Lord Mayor" or "The Bishop of Norwich". The person who holds the title is the peer, not his son who is only so styled by courtesy.
On one further point there is certainty. Whether the Lord Chamberlain's ruling is correct or not, it is certain that the combination "The Lady X" is confined to the range of the peerage. It must not be applied to the wife of a baronet or a knight.