Constitutional convention (political custom)

A constitutional convention is an informal and uncodified tradition 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 describe. 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, and in some cases not at all.

Some constitutional conventions operate separately from or alongside written constitutions, such as in Canada since the country was formed with the enactment of the Constitution Act, 1867. In 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 In most states, however, many old conventions have been replaced or superseded by laws (called codification).

Historical entities often had strong emphasis on constitutional convention. For example the constitution of the Roman Republic was codified comparatively late in its development and relied for its functioning on traditions and a shared moral code called mos maiorum. In the Holy Roman Empire such important issues as who could elect the emperor were entirely uncodified before the Golden Bull of 1356 and remained subject to a certain degree of interpretation well afterwards.