The leader of the opposition is the counterpart of the prime minister in the main non-government party.
The position was gradually formalised over the course of the 20th century. It was recognised in the 1937 Ministers of the Crown Act, when a salary of £2,000 per annum was made available.
Generally, whoever occupies the leadership of the largest opposition party in the Commons becomes leader of the opposition.
However, anomalies can arise.
Between 1918 and 1922 there was some confusion as to which party - Labour or the Liberals - occupied the opposition front bench and so no clear figure emerged.
During the period of coalition government in the First World War, it fell to the leader of the Liberals, Henry Asquith, to oppose a Liberal Prime Minister, Lloyd George.
In the Second World War the Labour leader, Clement Attlee, joined the government and nominated two members of his own party to act as leader of the opposition.
The leader of the opposition is allocated an office in the corridor behind the Speaker's Chair.