Writs are legal documents which authorise the holding of a general election or by-election. When Parliament is dissolved prior to a general election, writs are issued by the monarch declaring the holding of an election in each constituency.
The issuing of this writ starts the electoral timetable which results in a general election. In the case of a by-election the writ is issued by the Speaker of the House of Commons.
In the House of Lords a Member may not take his or her seat until he or she has obtained a writ of summons which is a document calling Members of the Lords to Parliament.