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. As they are normally (but not necessarily) the leader of the largest party in the House of Commons, they also usually have power over their respective political party.