Courtesy Titles

A peer's wife and children are granted the use of certain titles, depending upon the rank of the peer. 

Peerage Type	Wife	Eldest Son*	Younger Son	Daughter
Duke	Duchess	Marquess	Lord <Firstname>  <Surname>	Lady <Firstname>  <Surname>
Marquess	Lady <Husband'sTitle>	Earl	Lord <Firstname>  <Surname>	Lady <Firstname>  <Surname>
Earl	Lady <Husband'sTitle>	Viscount	The Honourable <Firstname> <Surname>	Lady <Firstname>  <Surname>
Viscount	Lady <Husband'sTitle>	The Honourable <Firstname> <Surname>	The Honourable <Firstname> <Surname>	The Honourable <Firstname> <Surname>
Baron	Lady <Husband'sTitle>	The Honourable <Firstname> <Surname>	The Honourable <Firstname> <Surname>	The Honourable <Firstname> <Surname>

* Eldest sons (heirs) of earls, marquesses, and dukes are allowed to adopt their father's next-highest title as a courtesy, which they use in every way as if it were a "real" peerage.

Eg: The Duke of Nerl's heir is known as the Marquess of Nerl, which is the duke's highest subordinate title. His subordinate titles are distributed by courtesy only to his direct heirs, that is, his eldest son, and his eldest son's eldest son, etc.