Canada's federal system was established under the British North America Act, initially uniting three provinces. The constitution allocates powers between the federal and provincial governments. It gives the federal parliament authority over 29 subjects in section 91, and allocates 16 local subjects to provincial legislatures in section 92. Constitutional amendments require the approval of both houses of parliament and two-thirds of the provinces representing at least 50% of the population. The judiciary is led by the Supreme Court of Canada, with federal and provincial appellate courts below it.