The Constitution of Canada is the supreme law of the country, and consists of written text and unwritten conventions. The ''Constitution Act, 1867'' (known as the British North America Act prior to 1982), affirmed governance based on parliamentary precedent and divided powers between the federal and provincial governments. The Statute of Westminster 1931 granted full autonomy, and the ''Constitution Act, 1982'' ended all legislative ties to Britain, as well as adding a constitutional amending formula and the ''Canadian Charter of Rights and Freedoms''. The ''Charter'' guarantees basic rights and freedoms that usually cannot be over-ridden by any government—though a notwithstanding clause allows Parliament and the provincial legislatures to override certain sections of the ''Charter'' for a period of five years.