Constitutional monarchy is a form of democratic government in which a monarch acts as a nonpolitical head of state within the boundaries of a constitution, whether written or unwritten. While the monarch may hold formal reserve powers and government may officially take place in the monarch's name, they do not set public policy or choose political leaders. Political scientist Vernon Bogdanor, paraphrasing Thomas Macaulay, has defined a constitutional monarch as "a sovereign who reigns but does not rule." This form of government differs from absolute monarchy, in which the monarch controls political decision-making and is not effectively restricted by constitutional constraints.
In addition to acting as a visible symbol of national unity, a constitutional monarch may hold formal powers such as dissolving parliament or giving Royal Assent to legislation. However, the exercise of such powers is generally a formality rather than an opportunity for the sovereign to enact personal political preference. In The English Constitution, British political theorist Walter Bagehot identified three main political rights which a constitutional monarch could freely exercise: the right to be consulted, the right to advise, and the right to warn.
The United Kingdom and fifteen of its former colonies are constitutional monarchies with a Westminster system of government. Three states—Malaysia, Cambodia, and the Holy See—employ true elective monarchies, where the ruler is periodically selected by a small, aristocratic electoral college.