The House of Lords is the upper house of Parliament - but this does not mean that it has more political clout than the Commons.
Members of the House of Lords, or peers, may be appointed because they have attained a degree of expertise in areas useful to the effective examination of government policy, such as science, economics or public administration.
Like the Commons, the House of Lords scrutinises all UK legislation except policy areas that have been devolved to the institutions in Scotland, Wales or Northern Ireland.
But since the Parliament Act was passed in 1911, MPs have been able to over-ride any decision made in the Lords, effectively giving it a subordinate role in the legislative process.
In practice, the Lords functions as a revising chamber, poring over the finer points of legislation and frequently sending laws that it finds objectionable back to the Commons for reconsideration.
Regular question sessions are held, giving backbench and opposition peers the opportunity to challenge government ministers.
Specific short debates and longer, more general debates on a wide range of subjects are also held throughout the week.
The chamber is easily identifiable by its red leather seats, as opposed to the green leather of the Commons.
Traditionally, the monarch comes to the House of Lords to outline the government's legislative proposals in the Queen's (or King's) Speech.
The House of Lords had 1,144 members until 1999, when 666 hereditary peers lost the automatic right to sit and vote in Parliament under Stage One of the Lords reform process.
This leaves 92 hereditary peers remaining in the upper chamber, while the government attempts to broker a deal on the second stage of its reform plans.
26 peers are drawn from the Church of England - known as Lords Spiritual, this group includes the Archbishops of Canterbury and York, with the remaining seats being taken by senior bishops.
Most members of the House of Lords are appointed by the prime minister and described as life peers - although a certain number of nominations from the leader of the opposition are also accepted.
In total, there are currently 746 peers.
At present, it is not possible to be expelled from the House of Lords - nor is it possible to resign.
The Lords and Commons began to sit as separate Houses in the 14th century with members drawn from the Church and others chosen by the monarch from the ranks of the aristocracy.
In the Middle Ages, the Church accounted for about half the membership.
The 1911 Parliament Act was brought in after the Conservative majority of hereditary peers - who were mostly wealthy land-owners - repeatedly rejected the Liberal government's budget of 1909 with its radical redistributive aims.
It stopped the Lords amending finance bills and reduced its ability to delay other bills. These restrictions were tightened further in 1949.
Under the Salisbury Convention which followed World War II, peers refrain from rejecting manifesto commitments on the grounds that such pledges have been approved by the electorate.
Unlike the House of Commons, where MPs are called to order by the Speaker and business and proceedings are regulated from the chair, the House of Lords operates a system of self-regulation.
At all times it is the decision of the House as to what business is to be discussed, how long it will be discussed for and who can and cannot speak and who is judged to be in or out of order.
Peers who talk too long or stray from protocol are often brought to heel with cries of "Order, order!" from their fellow peers on all sides.
The Constitutional Reform Act 2005 created the position of Lord Speaker to preside over debates - but the system of self-regulation continues as the lord Speaker has only nominal powers.
Baroness Hayman was elected the first Lord Speaker on 4 July 2006.