Courtesy peer

A courtesy title is a form of address in systems of nobility used for children, former wives and other close relatives of a , and by certain officials such as some judges. These styles are used 'by courtesy' in the sense that the relatives or officials do not themselves hold substantive titles. There are several different kinds of courtesy titles in the British peerage.

If a peer of one of the top three ranks (a duke, marquess or earl) has more than one title, his eldest son - not himself an actual peer - may use one of his father's lesser titles "by courtesy". However, the father continues to be the substantive holder of the peerage title and the son using the peerage by courtesy legally remains a commoner unless issued a writ of acceleration. If the eldest son of a duke or marquess has an eldest son, he may use a still lower title if one exists.

For example, the Duke of Norfolk is also Earl of Arundel and Baron Maltravers. His eldest son is therefore styled "Earl of Arundel" (without the definite article "The" which indicates a substantive title). Lord Arundel's eldest son (should he have one during his father's lifetime) would be styled "Lord Maltravers". However, only The Duke of Norfolk is actually a peer; his son Lord Arundel and his hypothetical grandson Lord Maltravers remain commoners.

Courtesy peerages are only used by the peer's eldest living son, and the eldest son's eldest living son, and so forth. Other descendants are not permitted to use the peer's subsidiary titles. Only the heir apparent (and heir apparent to the heir apparent, and so on) may use the titles. An heir presumptive (e.g., a brother, nephew, or cousin) does not use a courtesy title. However, Scottish practice allows the style Master/Mistress of X to an heir presumptive as well as to an heir apparent; for example, the brother of the present Marquess of Tweeddale has the title Master of Tweeddale.

...
Wikipedia