The decision by Prime Minister Malcolm Turnbull to call a double dissolution election represents the seventh occasion in Australia’s federal parliamentary history that this has happened. The last double dissolution was nearly 30 years ago.

A ‘double dissolution’ is a constitutional mechanism that allows a government (which has a majority in the House of Representatives) to overcome the blocking power of the Senate. The Australian Senate is powerful and can reject a Bill (a proposed law) even if it has passed in the House of Representatives.

The constitutional power to call a double dissolution is an Australian mechanism, a power created for the Governor-General, not an inherited reserve power of the British Monarch.

The term ‘dissolution’ refers to the act of dissolving something. A double dissolution ‘dissolves’ both Houses of Parliament – the Representatives and the Senate – in order to try to resolve an issue through an election. This can only happen when a Bill has been rejected by the Senate, or fails to pass, or passes with amendments that are not acceptable to the government, after two attempts.

Under Section 57 of the Constitution, which covers double dissolutions, there must be an interval of three months before a Bill is put for the second time.

If a deadlock continues, then the Prime Minister can advise the Governor-General to dissolve both Houses with a view to calling an election.

Under the Constitution, each state of the Australian federation has an equal number of senators. Twelve senators represent each of the six states, elected for a period of six years. A system of rotation, however, ensures that half the Senate retires every three years. A double dissolution election means that all positions become vacant.

If an election is held and the Bill or Bills that triggered the double dissolution are still not passed by the Senate, then the Prime Minister can call a joint sitting of both Houses. As the House of Representatives has 150 Members compared to the Senate’s 76, this guarantees the numbers for the passage of the Bill. A joint sitting has only happened once in Australia, in 1974.

There have been six double dissolutions during Australia’s 115 years as a federation. These were in 1914, 1951, 1974, 1975, 1983 and 1987.

It’s risky for a government to call a double dissolution. Of the six held, half have resulted in the return of the government. Even then, there is still no guarantee of a cooperative Senate.