Constitutional convention (political custom)

A constitutional convention is an informal and uncodified procedural agreement that is followed by the institutions of a state. In some states, notably those Commonwealth of Nations states that follow the Westminster system and whose political systems derive from British constitutional law, most government functions are guided by constitutional convention rather than by a formal written constitution. In these states, actual distribution of power may be markedly different from those the formal constitutional documents describes. In particular, the formal constitution often confers wide discretionary powers on the head of state that, in practice, are used only on the advice of the head of government.

Some constitutional conventions operate separate from or alongside written constitutions, such as in Canada since the country was formed with the enactment of the Constitution Act, 1867. Others, notably the United Kingdom which lack a single overarching constitutional document, unwritten conventions are still of vital importance in understanding how the state functions. In most states, however, many old conventions have been replaced or superseded by laws (called codification).

The term was first used by British legal scholar A. V. Dicey in his 1883 book, Introduction to the Study of the Law of the Constitution. Dicey wrote that in Britain, the actions of political actors and institutions are governed by two parallel and complementary sets of rules:

The other set of rules consist of conventions, understandings, habits, or practices that—though they may regulate the conduct of the several members of the sovereign power, the Ministry, or other officials—are not really laws, since they are not enforced by the courts. This portion of constitutional law may, for the sake of distinction, be termed the "conventions of the constitution", or constitutional morality.

A century later, Canadian scholar Peter Hogg wrote,

Constitutional conventions arise when the exercise of a certain type of power, which is not prohibited by law, arouses such opposition that it becomes impossible, on future occasions, to engage in further exercises of this power. For example, the constitutional convention that the Prime Minister of the United Kingdom cannot remain in office without the support of a majority of votes the House of Commons is derived from an unsuccessful attempt by the ministry of Robert Peel to govern without the support of a majority in the House, in 1834-1835.

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