In parliament a three line whip is said to be applied when a party seeks to ensure every MP turns up and votes the party line.
But why the term "three line" whip? And is there such a thing as a one or two line whip?
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There are indeed one and two line whips - they basically boil down to how strong the expectation to vote is.
A single whip is simply a guideline, while a double whip (or two line whip) is stricter - and attendance at the vote is required. The three-line whip is a 'vote with the party or get out'.
The number of lines comes from the number of times that a vote is underlined by the Chief Whip in the parliament's schedule.