A by-election takes place when a seat in the House of Commons becomes vacant between general elections.
Reasons for by-elections
A seat becomes vacant during the lifetime of a Parliament either when an MP resigns from Parliament, for example to take up a job which by law cannot be done by an MP, or because an MP has died. The law also allows a seat to be declared vacant because of a Member's bankruptcy, mental illness or conviction for a serious criminal offence.
A by-election does not automatically take place if an MP changes political party.
Until a new MP is elected, constituency matters are handled by an MP of the same party in a neighbouring constituency.
'Moving the Writ'
Traditionally the Chief Whip of the political party whose MP held the vacant seat will begin the procedure for a by-election. This is known as 'moving the Writ' and takes the form of a motion in the House of Commons.
A new Writ is moved within three months of the vacancy occurring. There have been a few instances of seats remaining vacant longer than six months before a by-election was called. Seats have also been left vacant towards the end of a Parliament to be filled at the general election.
If there are several vacant seats then a number of by-elections can take place on the same day.
House of Commons Library analysis
The House of Commons Library produces briefing papers to inform MPs of key issues. The papers contain factual information and a range of opinions on each subject, and aim to be politically impartial.