Triennial Act
1641 An Act of Parliament reluctantly agreed to by Charles I (who said it reduced his sovereign powers) which stated that there had to be a parliament of at least 50 days duration every three years.

It was designed to prevent a repetition of the long absences of parliament seen in the '11 Years Tyranny' of 1629-1640.

In fact, it was never used, and parliaments after the Civil War greatly moderated it, but it at least recognised the importance of parliament in the government of the country.