A subsidiary title is an hereditary title held by a royal or a noble but which is not regularly used to identify that person.

For example, the Duke of Norfolk is also the Earl of Arundel, the Earl of Surrey, the Earl of Norfolk, the Baron Beaumont, the Baron Maltravers, the Baron FitzAlan, the Baron Clun, the Baron Oswaldestre, and the Baron Howard of Glossop. In day-to-day practice, the individual who holds all of these titles would be referred to only by his most senior title - in this case, "Duke of Norfolk" - while all of his other titles would be subsidiary titles.

In the United Kingdom, a noble's heir apparent might use his father's most senior subsidiary title as a courtesy title. For example, the Duke of Norfolk's heir apparent might be called "Earl of Arundel" as a courtesy, although the son does not technically become Earl of Arundel until his father's death and is legally still a commoner.

Heirs apparent might also be summoned to the House of Lords before their fathers' death through the use of a writ of acceleration. In this situation, the heir was summoned as one of his father's subsidiary titles. (Usually, when a writ of acceleration was used, the son was summoned to one of his father's baronies.) Thus, on the above example, the Duke of Norfolk's son who had been using the Duke's subsidiary title of "Earl of Arundel" as a courtesy title might now be summoned to the House of Lords on the basis of his father's title of "Baron Maltravers" and would then actually sit in the House of Lords as "Lord Maltravers".