What I have set out above describes the general usage in the matter of courtesy titles for the heirs of peers. But until quite recently there had been little definite guidance on the subject and a pretty wide latitude has been tolerated and exercised, though each family usually has its established custom. Some families with several secondary titles alternate between two as the style for the heir.
Again, when an eldest son has died during his father's lifetime his son - or his brother, if a brother be the next heir - may decide to use another title rather than adopt the style used by the dead heir. Thus the present Duke of Westminster, who was known as Viscount Belgrave while second heir to the dukedom, continued that style after the death of his father, Earl Grosvenor.
The first-born son of the present Earl of Rosebery, after his father's succession to the peerage, was, following family custom, known as Lord Dalmeny, although the full style of the barony to which reference is made is Dalmeny and Primrose. But when Lord Dalmeny died in 1931 his infant brother, who then became heir, was given the courtesy style of Lord Primrose. Primrose is also the family name. Similarly, when the sixth Marquess of Lansdowne's eldest son, known as the Earl of Kerry, died in 1933, the second son, who then became heir, adopted the courtesy style of Earl of Shelburne, another of Lord Lansdowne's peerages.
The eldest son of the Earl of Clanwilliam is normally known as Lord Gillford, which is a barony attached to the earldom. But when the eldest son of the late Earl died, having held that courtesy style, his brother, the next heir (now the present Earl), was known by the purely imaginary title of Lord Dromore, a device arranged between the Earl and the late King Edward VII.
Generally it is in the case of brother succeeding brother as heir that this adoption of a different style is resorted to. The usual practice is for a son to adopt his father's style.
But, as I have said, so much latitude has been allowed in regard to courtesy titles, and the practice as between different families varies so much, that in considering particular cases it is desirable to refer to such standard peerage works as Burke's or Debrett's, which form reliable guides.
It should not be necessary to explain that Christian names can never be inserted between the rank and the name of a peerage, whether referring to the actual holder or an heir who is using it as a courtesy title. That fact is elementary, but it is so often infringed both in newspapers and in books that it clearly needs emphasising. It is impossible to speak or write of "Viscount Robert Edward Peter Cranborne" in referring to Lord Salisbury's heir. No such style is known in British peerage usage. If one wishes to set out his names it must be in the form of "Robert Edward Peter, Viscount Cranborne".