Brewer's: Courtesy Titles
Titles assumed or granted by social custom, but not of any legal value. The courtesy title of the eldest son of a duke is marquis; of a marquis is earl; of the eldest son of an earl is viscount. Younger sons of peers are by courtesy called lord or honourable, and the daughters are lady or honourable. These titles do not give the holders official rank to sit in the House of Lords. Even the Marquis of Lorne, the Queen's son-in-law, is only a commoner (1894).