In British law, a commoner is someone who is neither the Sovereign nor a peer. Therefore, any member of the Royal Family who is not a peer, such as HRH Prince William of Wales or HRH The Princess Royal, is a commoner, as is any member of a peer's family, including someone who holds only a courtesy title, such as the Earl of Arundel and Surrey (eldest son of the Duke of Norfolk) or Lady Victoria Hervey (a daughter of the 6th Marquess of Bristol).
Traditionally, members of the House of Commons were commoners, though the name of the House of Commons comes from the communities they represent not their rank, while members of the House of Lords were peers. Peers whose only titles are in the Peerage of Ireland have been able to stand for election to the House of Commons for centuries. Since the House of Lords Act 1999, which excluded most hereditary peers from the House of Lords, most hereditary peers can now stand for election to the House of Commons. For example, the 13th Marquess of Lothian (aka Michael Ancram), the 3rd Viscount Thurso (aka John Thurso) and the 3rd Viscount Hailsham (aka Douglas Hogg) are currently members of the House of Commons.
