Examples of Courtesy Titles
The eldest (or only) son and heir apparent of a duke, marquess or earl may by courtesy use a title in the peerage of a rank junior to his father. This is usually, but not invariably, the second senior peerage borne by the peer.
There is no hard and fast rule about which title borne by a peer is selected for use by his eldest son as a peer by courtesy, though in most families custom is followed.
If the Marquess of Lansdowne has a son, his heir is usually known as Earl of Kerry or Earl of Shelburne in alternate generations.
The late Earl of Wemyss and March, after the untimely deaths of successive Lords Elcho, decided that his only surviving son should use the courtesy peerage title of Lord Neidpath instead of Elcho.
When the 11th Duke of Devonshire died in 2004, he was succeeded in that dignity by his only son, previously known by courtesy as the Marquess of Hartington. The new Duke of Devonshire's son, however, has not assumed the title of the Marquess of Hartington, preferring to retain the use of his courtesy title of Earl of Burlington (traditionally the title of the grandson of the Duke of Devonshire).
It is very important that anyone corresponding with a member of the peerage is aware of the rank and precedence of the person he or she is in contact with, so that the correct form of address may be used.