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. It has been criticised by some as either a move designed to intimidate smaller opponents, or to take control of the upper house of parliament. Only six times since the formation of the Commonwealth of Australia over 110 years ago has the Constitutional measure been used. Takes a look at what a double-dissolution election is, and its place in Australian political history.