In the British peerage system, wives, children, and other close relatives of a peer are addressed by styless that may mislead those unacquainted with the system into thinking that they have substantive titles.
If a peer of the rank of Earl, Marquess, or Duke has more than one title, his eldest son uses one of the lesser titles. (The eldest sons of Barons and Viscounts do not receive such a privilege.) If that eldest son has an eldest son, and there are additional titles available, he too may use a lesser title. For example, the Duke of Norfolk is also the Earl of Arundel and Lord Maltravers, and so his son may be styled Earl of Arundel, and the grandson styled Lord Maltravers. However, only the grandfather is a peer: the other two remain 'commoners' until they actually acquire a substantive title. Also, such courtesy titles are only used by the peer's eldest son, and the eldest son's eldest son, and so forth. Other descendants are not permitted to use the peer's subsidiary titles.
The actual title used is a matter of family tradition. For instance, the eldest son of the Duke of Buccleuch and Queensberry is the Earl of Dalkeith, even though the Duke is also the Marquess of Dumfriesshire, a senior title to the Earldom of Dalkeith. Similarly, the eldest son of the Marquess of Londonderry is Viscount Castlereagh, even though the Marquess is also the Earl Vane. Titles with the same name as a peer's main title are also not used as courtesy titles. For instance, the Duke of Westminster is also the Marquess of Westminster and the Earl Grosvenor (amongst other things). The Duke's son is not the Marquess of Westminster (which would cause confusion between the son and the father), and so is styled Earl Grosvenor instead. The title used does not have to be exactly equivalent to the actual peerage: the eldest son of the current Duke of Wellington uses the title "Marquess of Douro", even though the actual peerage possessed by his father is "Marquess Douro". If a peer of the rank of Earl or above does not have any subsidiary titles of a different name to his main title, his eldest son usually uses an invented courtesy title of "Lord Surname". For instance, the eldest son of the Earl of Devon is Lord Courtenay, even though the Earl has no barony of that name, and similarly the eldest son of the Earl of Guilford is Lord North. The eldest son of the Earl of Huntingdon, who has no subsidiary titles, is styled Viscount Hastings to avoid confusion with the substantive peer Lord Hastings.
A peer's wife takes her courtesy title based on her husband's rank, unless she herself has a higher title. Thus a baron's wife is called "baroness", an earl's wife is called a "countess", a duke's wife a "duchess", etc. Despite being referred to as a "peeress", she does not, however, become a peer "in her own right": these are 'styles', not substantive titles.
Another form of courtesy title, in the form of an honorific prefix, is granted to younger sons, and all daughters of peers. The rules differ for different ranks of peers: the children of a baron, for example, get the prefix "Hon.", the daughters of an earl are called "Lady", and so on:
|Peer||Wife||Eldest Son||Younger Son||Unmarried Daughter|
|Duke||Duchess||Father's Subsidiary Title||Lord [Firstname] [Lastname]||Lady [Firstname] [Lastname]|
|Marquess||Marchioness||Father's Subsidiary Title||Lord [Firstname] [Lastname]||Lady [Firstname] [Lastname]|
|Earl||Countess||Father's Subsidiary Title||The Honourable [Firstname] [Lastname]||Lady [Firstname] [Lastname]|
|Viscount||Viscountess||The Honourable [Firstname] [Lastname]||The Honourable [Firstname] [Lastname]||The Honourable [Firstname] [Lastname]|
|Baron||Baroness||The Honourable [Firstname] [Lastname]||The Honourable [Firstname] [Lastname]||The Honourable [Firstname] [Lastname]|
Note that a peer's daughter that marries a commoner retains her courtesy title (substituting her husband's surname for the maiden name), but if she marries a peer, she gains the courtesy title as that peer's wife. Also note that the children of a peeress in her own right (a peeress that holds a substantive title, and is not merely a wife of a peer) gain courtesy titles as usual, but the husband receives no special distinction.