Because legislation must pass through both houses to become law, it is possible for disagreements between the House of Representatives and the Senate to hold up the progress of government bills indefinitely. Such deadlocks can be resolved through section 57 of the Constitution, using a procedure called a double dissolution election. Such elections are rare, not because the conditions for holding them are seldom met, but because they can pose a significant political risk to any government that chooses to call one. Of the six double dissolution elections that have been held since federation, half have resulted in the fall of a government. Only once, in 1974, has the full procedure for resolving a deadlock been followed, with a joint sitting of the two houses being held to deliberate upon the bills that had originally led to the deadlock.