The Australian electoral system comprises the laws and processes used for the election of members of the Australian Parliament and is governed primarily by the Commonwealth Electoral Act 1918. The system presently has a number of distinctive features including compulsory enrolment; compulsory voting; majority-preferential instant-runoff voting in single-member seats to elect the lower house, the House of Representatives; and the use of the single transferable vote proportional representation system to elect the upper house, the Senate. The timing of elections is governed by the Constitution and political conventions. Generally, elections are held approximately every three years and are conducted by the independent Australian Electoral Commission (AEC). Conduct of elections Federal elections, by-elections and referendums are conducted by the Australian Electoral Commission (AEC). Voter registration In Australia, voter registration is called enrolment, which is a prerequisite for voting at federal elections, by-elections and referendums. Enrolment is compulsory for Australian citizens over 18 years of age who have lived at their current address for at least one month. Residents in Australia who had been enrolled as British subjects on 25 January 1984, though not Australian citizens, continue to be enrolled. They cannot opt out of enrolment, and must keep their details updated, and vote. (These comprise about 9% of enrolments.) Electoral rolls The AEC maintains a permanent Commonwealth electoral roll. State and local elections are today based upon the Commonwealth electoral roll, maintained under joint roll arrangements, though each state and territory regulates its own part of the electoral roll. The same enrolment application or update form can be used for Commonwealth, state and local rolls. A protection in Section 101 (8) exists for offences prior to enrolment (including failure to enrol) for those enrolled in such a way by the Electoral Commissioner. Anyone serving a prison sentence of 3 years or more is removed from the federal roll, and must apply to re-enrol upon release. Enrolment of electors Special rules apply to citizens going or living outside the country, to military personnel and to prisoners, all of which do not reside at their normal residential address for electoral purposes. Homeless people or those otherwise with no fixed address have a particular problem with registration, not having a current address to give. Enrolment is optional for 16- or 17-year-olds, but they cannot vote until they turn 18. A person can register or update their details online or by mailing in a paper