Triennial Act of 1694

Stipulated that Parliament should be held within three years after the last Parliament was dismissed and that it could not last longer than three years. Therefore, according to this Act, the monarchs could not summon or dissolve Parliament whenever they wanted. The Triennial Act was vetoed twice by William III before it was passed.

Source

Smith, David L. A History of the Modern British Isles 1603-1707. Oxford: Blackwell, 1998.