A Constitutional Monarchy is a form of Constitutional government, where a hereditary Monarch is the Head of State, unlike in an Absolute Monarchy, wherein the King or the Queen is the sole source of political power, as he or she is not legally bound by the Constitution. The levels and types of power and authority held by the Monarch vary from case to case, as does the nature and guarantees of the Constitution. This is a system of government in which a Monarch shares power with a Constitutionally organized government, where the Monarch may be the defacto Head of State or a purely ceremonial leader. The Constitution allocates the rest of the government’s power to the legislature and judiciary. The United Kingdom is a Constitutional Monarchy, where its succession to the British throne is hereditary, no longer governed by the principle of male preference, but excludes Roman Catholics from ascending to the throne. Heirs to the throne can now marry Roman Catholics with out renouncing their claim to the throne. Under the British Constitution, sweeping executive powers, known as the royal prerogative, are nominally vested in the Sovereign. In exercising these powers, however, the Sovereign normally defers to the advice of the Prime Minister or other ministers. This principle, which can be traced back to the Restoration, was most famously articulated by the Victorian writer Walter Bagehot as: “the Queen reigns, but she does not rule”.