Prime minister's questions, or PMQs, is a high point of the parliamentary week.
A to Z: Prime Minister's Questions
Each week, at noon on Wednesday the prime minister must come to the House of Commons to answer oral questions for half an hour.
The current arrangements for PMQs began after the Labour government came to power in May 1997.
Previously PMQs ran for 15 minutes and took place on Tuesday and Thursday afternoons.
The leader of the opposition may make up to six interventions (previously three) during PMQs. When not in a coalition government the Liberal Democrats' leader normally asks two questions.
Labour argued that one half-hour session was better than two shorter sessions, as backbench MPs would have more opportunity to question the prime minister in depth.
The session always starts with a routine question about the prime minister's engagements for the day.
MPs are not obliged to give the prime minister prior notice of the subjects that they are going to raise in all subsequent questions.
This element of surprise allows opposition MPs in particular to try to catch the prime minister out with an awkward question.
The prime minister must respond without delay, thinking on his or her feet - but he or she is guaranteed the last word in any exchange.
Government backbenchers can normally be relied upon to ask "helpful" questions, often planted by the Whips, which will allow the prime minister to tell the House about successful government policies.
The relative performance of each of the main party leaders is closely watched and each is under great pressure to get the better of their opponent.
'Punch and Judy'
But critics question whether success at the despatch box imparts much influence outside the Westminster village.
Others argue that it is not an effective way of holding the government to account since backbenchers are not able to ask a supplementary question unless they give the PM prior notice of their chosen subject.
And although the leaders of the two largest opposition parties can ask supplementary questions, the tone of such exchanges is often likened to "Punch and Judy" politics.
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