Many modern presidential republics employ term limits for their highest offices. The United States placed a limit of two terms on its presidency by means of the 22nd Amendment to the Constitution in 1951. There are no term limits for Vice Presidency, Representatives and Senators, although there have been calls for term limits for those offices. Under various state laws, some state governors and state legislators have term limits. Formal limits in America date back to the 1682 Pennsylvania Charter of Liberties, and the colonial frame of government of the same year, authored by William Penn and providing for triennial rotation of the Provincial Council, the upper house of the colonial legislature. (See also term limits in the United States).