The Peerage of the United Kingdom comprises most peerages created in the United Kingdom of Great Britain and Ireland after the Acts of Union in 1801, when it replaced the Peerage of Great Britain. New peers continued to be created in the Peerage of Ireland until 1898 (the last creation being the Barony of Curzon of Kedleston).
The last non-royal dukedom was created in 1900, and the last marquessate in 1936. Creation of the remaining ranks mostly ceased once Harold Wilson's Labour government took office in 1964, and only four non-royal hereditary peerages have been created since then. These were:
Until the House of Lords Act 1999 was passed, all peers of the United Kingdom were automatically members of the House of Lords. However, from that date, most of the hereditary peers ceased to be members as part of Parliamentary reform, whereas the life peers retained their seats. All hereditary peers of the first creation (i.e., those for whom a peerage was originally created, as opposed to those who inherited a peerage from an ancestor), and all surviving hereditary peers who had served as Leader of the House of Lords were offered a life peerage in order to allow them to sit in the House should they wish.
Marquesses, earls, viscounts and barons are all addressed as 'Lord X', where 'X' represents either their territory or surname pertaining to their title. Marchionesses, countesses, viscountesses and baronesses are all addressed as 'Lady X'. Dukes and duchesses are addressed just as 'Duke' or 'Duchess' or, in a non-social context, 'Your Grace'.