voter is given as many votes as there are candidates to be elected. From 1929, this was changed to instant-runoff voting, with multi-member districts using the preferential block voting variant of the system. In 1973, as part of the reforms to the upper house, the Legislative Council switched to using a party list form of proportional representation voting that was subsequently changed to the single transferable vote in 1982. From 1985 until 2017, the Legislative Council also used a form of Group voting tickets similar to that used in the Senate at the time. This enabled voters to choose between voting 'above the line' (for a single party preference ticket that ranked all candidates on the ballot paper in the party's preferred order) or 'below the line' and number all candidates in order of preference, on the ballot paper. In 2017 this was changed to a form of optional preferential voting − instructions for above the line votes are to mark '1' and then further preferences are optional as opposed to preference flows from simply '1' above the line being determined by group voting tickets, while instructions for voters who instead opt to vote below the line are to provide at least 12 preferences as opposed to having to number all candidates, and with a savings provision to admit ballot papers which indicate at least 6 below the line preferences. Today, the House of Assembly continues to use instant-runoff voting with compulsory preferences. Compulsory voting was introduced for the House of Assembly in 1942 and for the Legislative Council in 1985. Elections were held every 3 years until 1985 (with a brief flirtation with 5 year terms for the House of Assembly in the 1933-1938 parliament), when the parliament switched to 4 year terms, meaning 8 year terms for the upper house. Beginning in 2006, election dates have been fixed at the third Saturday in March of every fourth year. House of Assembly The House of Assembly (or "lower house") is made up of 47 members who are each elected by the full-preference instant-runoff voting system in single-member electorates. Each of the 47 electoral districts (electorates) contains approximately the same number of voters. Casual vacancies, when a member retires or dies in office mid-term, are filled by a by-election in that member's district. Since 1975, the distribution of electoral boundaries has been set by the South Australian Electoral Districts Boundaries