Heirs in the Direct Line
Courtesy titles fall, broadly speaking, into two classes - those in the style of peerages, which are borne only by heirs in the direct line; and those, consisting of the prefixes "Lord", "Lady" or "the Honourable" to the Christian name, which are borne by other sons and daughters.
We will deal with the former first. In courtesy the eldest son of a duke is born with the precedence of a marquess, the eldest son of a marquess with that of an earl, the eldest son of an earl with that of a viscount. (Eldest sons of viscounts and barons will be considered later.) This in each case is their actual precedence, even though there may not be equivalent titles available for their use.
We may take as an example of the fullest use of this system of courtesy titles under the case of the Dukedom of Devonshire. The son and heir is known as Marquess of Hartington, and his elder son as Earl of Burlington - both minor titles held by the Duke. Four generations alive in the direct line must be exceedinly rare, but if it ever did occur in the Devonshire family presumably the great-grandson would be known as Lord Cavendish, one of the Duke's baronies, for there is no viscounty.
But not all dukes hold marquessates also, and in those cases the eldest son takes as his courtesy title whatever is his father's secondary title. In all these cases except one (that of Somerset) this is an earldom. Thus the eldest son of a Duke of Grafton is known as Earl of Euston and his son as Viscount Ipswich.
The Dukedom of Somerset is alone in having only a barony attached to it. There have been earldoms and baronies a-plenty to go with this dukedom, but in the vicissitudes of descent all have been extinguished, forfeited or become separated from the dukedom, until now only the Barony of Seymour survives - in striking contrast to, for instance, the Dukedom of Atholl, the holder of which has altogether nineteen peerages.
But there is one exception even to these cases where the secondary ducal title is an earldom, and that is the Dukedom of Manchester. The only earldom held by the Duke is also of Manchester. To have a Dukedom of Manchester and an Earldom of Manchester in use simultaneously is asking for confusion. In this case, therefore, the heir forgoes his father's earldom and is known as Viscount Mandeville, which is the Duke's third title.
In the case of a real viscount, i.e. a peer of that rank, it has never been the custom for a son to use the courtesy title of "Lord", however many baronies his father possessed in addition to his viscountcy, but as Lord Mandeville's use of that style is merely a matter of convenience, and his precedence is that of a marquess, his son is allowed as a courtesy title the use of his grandfather's barony, and is known as Lord Kimbolton. In this case the son's daughters are also accorded the courtesy style of a marquess's children and bear the "Lady" prefixed to their Christian names. But this practice is not general.
Heirs to Marquessates
The same general rules apply to the direct line of heirs to a marquessate. Where the title is available the eldest son is known by the style of his father's earldom, and his eldest son by that of the grandfather's barony. Thus the heir to the Marquess of Bute is Earl of Dumfries, while his son is Lord Cardiff.
But here again there are exceptions due to the absence of an earldom in the family honours. Thus the elder son of the Marquess of Bath is known as Viscount Weymouth. In this and other cases it is not the custom for the grandson to be known by the style of one of the family baronies. They are styled "the Honourable" as a prefix to Christian and surnames as is the case with the eldest son of a viscount who is a peer.
Among the marquesses there are exceptions on the same ground as that instanced in the Manchester Dukedom. The Marquesses of Exeter, Salisbury, Queensberry, Reading and Willingdon all hold earldoms of the same name, so that in each case the heir uses the courtesy title of viscount.
Another exception is the Marquess of Londonderry, for though he is also Earl Vane the heir is always known by the style - a historic one - of Viscount Castlereagh.
The late Lord Londonderry told me that the reason for this is twofold. One is the desire to preserve the use of the Castlereagh title. The other was because it is by virtue of his Earldom of Vane, a United Kingdom one, that Lord Londonderry sat in the House of Lords, and that as he himself thus used the earldom for official purposes - he was summoned to the House in it, he appeared in the division lists in it, and signed himself by it in the House when occasion arose - there was a doubt about it being available as a courtesy style for the heir. On this latter point Lord Londonderry tried to get a definite ruling some thirty years ago, but without success.
Lord Londonderry's story of an ancestor's plans for establishing a second noble line in England while the original line remained domiciled in Ireland is interesting. The third Marquess, head of the family of Stewart of Mount Stewart in Co. Down, was Marquess and Earl of Londonderry, Viscount Castlereagh and Baron Londonderry, all in the peerage of Ireland, but with a United Kingdom barony of Stewart, of Stewart's Court and Ballylawn, which gave him a seat in the House of Lords. By his first marriage he had a son who, he hoped, would provide heirs to carry on this Irish line.
By his second marriage to Lady Frances Vane-Tempest, heiress to the great Vane-Tempest estates in Durham, he had other children, and when subsequently he secured the United Kingdom Earldom of Vane and the Viscountcy of Seaham it was with special remainder to the heirs male of this second marriage, his hope being that they would carry on the Vane-Tempest line and tradition separately from the Stewart line.
Unfortunately for this design the eldest son did not marry until rather late in life and had no children, so all the titles eventually came to his half-brother, who succeeded as fifth Marquess.
The half-brother himself seems to have resented this interference of Providence with his sire's plans, for thereafter, Lord Londonderry told me, he always signed himself "Vane-Londonderry", for which variation from the normal there was not, of course, the slightest justification.
"So you see", added Lord Londonderry, "it was never intended that I should be Marquess of Londonderry at all. I was to be Earl Vane, with a more or less distant cousin in Ireland as Lord Londonderry."
Nevertheless, despite this unintentional accretion of an earldom, the style of Viscount Castlereagh has always been retained for the heir, and, since this is a matter of choice and his precedence remains that of an earl, the eldest grandson is accorded the courtesy style which would be his were his father known as an earl, and is styled Lord Stewart. (See earlier reference to Manchester Dukedom.)
But, oddly enough - and this is typical of a number of inconsistencies to be found in courtesy title practice - although the family custom is to give the second heir the style of an earl's son, his sisters have never used the style of "Lady", which on this reasoning should be theirs, but are always called "the Honourable".
Heirs to Earldoms
The eldest son of an earl takes his father's secondary title, whether that be a viscountcy or a barony. Thus the Earl of Harewood's elder son is Viscount Lascelles, while the Earl of Derby's is known as Lord Stanley, for Lord Derby holds no viscountcy.
Heirs to Viscountcies and Baronies
In the case of viscounts and barons (except for Scots peers of these ranks, which will be dealt with later) there is no distinction between elder and younger sons. All have "the Honourable" prefixed to their Christian and surnames.
"Invented" Courtesy Titles
It is opportune here to explain the custom of "inventing" courtesy titles when occasion warrants. This arises where a peer of a higher rank than viscount has no secondary title which can be borne by his son, the minor titles having by the accidents of descent become extinguished or alienated. Usually the difficulty has been overcome by prefixing Viscount or Lord to the family surname, where that is different from the title. The position at the present time exists only in the Earldoms of Devon, Huntingdon and Temple of Stowe. In the first case the heir is known as Viscount or Lord Courtenay, in the second as Viscount Hastings, and in the third as Lord Langton.
Until recently the Earldom of Lindsey was in the same category and the heir was known as Lord Bertie (the family name). But with the death of the late (twelfth) Earl the title passed to a remote cousin, the eighth Earl of Abingdon. As Lord Abingdon holds also the Barony of Norris (or Norreys) the difficulty does not at the moment exist. But the barony is one of writ, heritable in the female line, so that should this or any subsequent holder of the titles leave a daughter or sister as his immediate heir she would get the barony while the earldoms would go to the next male heir. In that case not one but both earldoms would be lacking a secondary title to serve as a courtesy title for the heir.