Courtesy title

In the context of nobility, a courtesy title is a title that is not a substantive title but rather is used through custom or courtesy.

In some contexts, courtesy title is used to mean the more general concept of a title or honorific such as Mr., Mrs., Ms., Dr., miss, sir, and madam.

France
In France, for example, many titles are not substantive titles but courtesy titles. A common practice is descending title when cadet males of noble families, especially landed aristocracy, may assume a lower courtesy title such as count even though lacking a titled seigneury themselves. For example the eldest son of the Duke of Paris (substantive title) may be called Marquess of Paris (courtesy title) and younger sons Count N. of Paris, where N. stands for the first name.

Courtesy title as principal title
During the Ancien Régime the only substantive titles were feudal, land-based, and resulted from a royal grant. In order to use the title of count, one had to own a seigneurie elevated to county and to comply with the remainder of the grant. However, these legal prescriptions were enforced in only the title that gave precedence and privileges, the title of duke. Most of the titles were courtesy titles, even used at Court and in legal documents, but not substantive titles.

Courtesy title used by sons and daughters
The heir apparent of a nobleman used a courtesy title from the lesser titles of his father. In the 17th century, the heirs of the most powerful dukes used the title of prince. In the 18th, a common trend was for the heir to use the title of duke. It was achieved by three ways. The head of family may have two dukedoms and his heir could use the junior one ; the head of family could resign his peerage to his heir, giving him a new title of duke while he kept his old title ; the king could confer a brevet de duc, that is style and precedence of a duke, to the heir of a title.

The younger sons of a nobleman used one of the lesser titles, but never one of duke or prince. Even in small gentry families with no title, every son used a different territorial designation, the so-called nom de terre. The clergymen before the episcopal ordination used the title of abbé, followed by the name of the principal title of their father. Members of the Order of Saint John used the title chevalier in the same fashion.

The daughters used the title of mademoiselle, followed by the name of a title held by their father.

For example, Anne Marie Louise d'Orléans, Duchess of Montpensier known as La Grande Mademoiselle, was the eldest daughter of Gaston d'Orléans (Monsieur) and his first wife Marie de Bourbon, Duchess of Montpensier. Anne Marie Louise was officially known as Mademoiselle from the time of her birth.

United Kingdom
There is a detailed system of styles and courtesy titles in the United Kingdom, by which the eldest son, male-line grandson or great-grandson and heir of a peer may use a subsidiary title of his ancestor even though it is the ancestor who holds the title substantively. By United Kingdom law, users of courtesy titles have nonetheless been held to be commoners, eligible for election to the House of Commons rather than the House of Lords.

Ireland
Article 40.2 of the republican Irish Constitution forbids the state conferring titles of nobility and a citizen may not accept titles of nobility or honour except with the prior approval of the Government. The continued conferring of United Kingdom Irish nobility titles on Irish citizens being considered anachronistic, all claims of nobility in Ireland are thus simply a courtesy title.