A Coronation Problem
Although convention had accorded these courtesy styles to such grand-daughters of peers for many years, they were not in fact officially recognised until the Coronation of the present King and Queen in 1937. It arose out of the choice of the six Maids of Honour to assist the Dowager Duchess of Northumberland, the Mistress of the Robes, to wait upon the Queen. When the official list was being prepared it was pointed out, by Garter King of Arms, that although five of their number were correctly given the style of "Lady" as the daughters of peers, the other, known conventionally as Lady Margaret Cavendish-Bentinck, was not officially entitled to that style as she was not the daughter of a peer but only of a peer's elder son (Marquess of Titchfield, heir to the Duke of Portland).
The King overcame the immediate difficulty by issuing at once a Royal Warrant expressly granting Lady Margaret the use of that style.
But this raised the whole question of courtesy titles and subsequently, in 1939, recommendations were submitted to the King designed to put the practice on a more regular basis.
Briefly, these recognised the use of courtesy peerage styles for the grandsons of dukes and marquesses in the direct line of succession, provided the titles were those of actual peerages held by the grandfathers; and allowed such a grandson to "succeed" to any higher courtesy style held by his father if his father died in the grandfather's lifetime.
But they also proposed not to allow officially courtesy titles to any other children of peer's eldest sons without the sanction of a Royal Warrant in each case. The effect of this would have been that though the eldest son of a courtesy marquis might correctly be known as Earl of This or That, his brothers and sisters would officially be merely Mr. and Miss.
The King's Decision
It appears, however, that His Majesty was loath to interfere to this extent with a practice which had become so general, and the upshot was a general permission for all the children of holders of courtesy peerage titles to bear such styles as would be theirs if their fathers actually held the peerages by which they were known.
Actually this latest ruling by the King refers only to the grandchildren of dukes and marquesses, but adherence to the spirit and not merely the wording of it would seem to rule out the further use of those "invented" courtesy titles to which I have referred. But as fairly long usage has sanctioned the custom, and the peerages concerned are few in number, any difficulty would probably be met by the issue of a Royal Warrant granting permission.