the next election. Such tables frequently appear in the Australian media and are called election pendulums or sometimes Mackerras pendulums after the political scientist Malcolm Mackerras, who popularised the idea of the two-party vote in his 1972 book Australian General Elections. Here is a sample of the federal election pendulum from the 2001 election, showing some of the seats held by the Liberal-National Party coalition government, in order of their two-party majority. A seat with a small two-party majority is said to be a marginal seat or a swinging seat. A seat with a large two-party majority is said to be a safe seat, although "safe" seats have been known to change hands in the event of a large swing. Redistributions The boundaries of Australian electoral divisions are reviewed periodically by the Australian Electoral Commission and redrawn in a process called redistribution. The Commonwealth Electoral Act 1918 requires that all seats have approximately an equal number of enrolled voters. When the Commission determines that population shifts within a state have caused some seats to have too many or too few voters, a redistribution is called and new boundaries are drawn up. Redistributions are also held when the Commission determines (following a formula laid down in the Electoral Act) that the distribution of seats among the states and territories must be changed because some states are growing faster than others. House casual vacancies If a member's seat becomes vacant mid-term, whether through disqualification, resignation, death or some other possible reason, a by-election may be held to fill the casual vacancy. A member may resign by tendering the resignation to the Speaker, as required by section 37 of the Australian Constitution, or in the absence of the Speaker to the Governor-General. A resignation is not effective until it is tendered in writing to the Speaker or Governor-General. If a redistribution has taken place since the last election, the by-election is held on the basis of the boundaries at the time of original election. Senate The Australian Senate has 76 members: each of the six states elects 12 senators, and the Northern Territory (NT) and the Australian Capital Territory (ACT) each elect two senators. The several other Australian Territories have very small populations and are represented by Northern Territory and ACT Senators (for example, Christmas Island residents are represented by NT Senators, while Jervis Bay Territory and Norfolk Island residents are represented by