What is a double dissolution election?

The Prime Minister's threat to Senate crossbenchers to either pass industrial relations legislation or possibly face a double-dissolution election may herald a rare political event. What is a double dissolution election?

Australian Prime Minister Malcolm Turnbull speaks to the media during a press conference at Parliament House in Canberra Monday, March 21, 2016. Source: AAP

1. In Australia the parliament comprises two chambers - the House of Representatives and the Senate (a house of review).

Parliament House - Canberra Source: PX

2. For a double-dissolution election to occur, the government needs what's known as a "trigger bill" (in this instance the trigger bills are the Registered Organisation Bill and the Australian Building and Construction Commission – ABCC Bill.)

Construction worker Source: PX

3. If the Senate refuses to pass the trigger legislation, both houses of parliament can be called for a joint sitting.

Parliament Sitting Source: Getty Images

4. If the joint seating brings no positive result we could be forced to an early double-dissolution election.

Ballot box Source: AAP

5. The first double-dissolution election was called in 1914 over union employment in the public service. The most recent was in 1987 - again by a Labor government, but this time using legislation proposing the introduction of a national identity card.

The proposed Australia Card that would function as a national identity card, 1987 Source: The Australian