The powers of the prime minister of the United Kingdom come from several sources of the UK constitution, including both statute and constitutional convention, but not one single authoritative document. They have been described as "...problematic to outline definitively." The UK has a fusion of powers, which means that the prime minister exercises functions in both the executive and the legislature. The prime minister normally (but not necessarily) leads the largest party in the House of Commons and they usually have some power over their own party due to that role.