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In politics, a casual vacancy (casual in the sense of "by chance") is a situation in which a seat in a deliberative assembly is vacated during that assembly's term. Casual vacancies arise through the death, resignation or disqualification of the sitting member.
Casual vacancies have the effect of eliminating or reducing representation for the member's constituency. Accordingly, many jurisdictions provide by law for the speedy filling of vacant seats.
As of 2008[update], the New Zealand House of Representatives has 70 seats whose members represent geographical constituencies, and a further 52 seats selected from party lists. A vacancy in any of the 70 "electorate seats" is filled through a by-election. The writ for the by-election must, in most circumstances, be issued within 21 days of the notification of the vacancy. A by-election is not required if Parliament is due to expire within six months.
By contrast, a vacancy in one of the "list seats" is filled by the next available candidate on the list submitted by the party holding that seat.
The phrase "casual vacancy" is not commonly used in the U.S. A vacancy in the House of Representatives is filled by a special election. The Constitution requires the executive authority of the state concerned to issue writs of election.
The seventeenth amendment provides that vacancies in the Senate are also filled by popular election, while allowing the legislature of the state concerned to pass laws authorising the state's governor to make a temporary appointment to the Senate until a popular election may be held.