A constitution is a set of rules that are the ultimate source of legal authority for the state. The rules of the constitution identify the major institutions of the state, and govern the relationship between the state and the individual citizen.
In most constitutions the constitution is codified in a single document: exceptions include the United Kingdom, New Zealand, and Israel.
Constitutions identify the principal institutions of the state, which, most commonly, the executive, the legislature, and the judiciary. The executive may, in parliamentary states, be a subset of the elected legislature. Some nations, such as the State of Iran, have much more complex institutions, with at least six major organizations.
The eminent constitutional lawyer Laurence H Tribe has suggested that formal documents necessarily provide an incomplete account of a country's constitution, and that their provision is everywhere supplemented by an "invisible constitution" consisting of generally-accepted conventions concerning their interpretation and concerning circumstances that they did not envisage.
Constitutions may be republican or monarchical. In a republic, the head of state is directly elected by the people or indirectly by a representative body elected by the people. Some republics, such as the United States, make the same individual head of state and head of government, while others, such as the state of Israel and France, separate the functions among two individuals. In a constitutional monarchy, the monarch is the titular head of state. In both cases, the power to amend the constitution rests with an elected assembly
In a federal state, the constitution ensures that the balance of power is diffused between powers that may be exercised by the central federal government and its constituent parts (usually states). Conflicts between federal and state concerns are resolved by referring to the constitution. A unitary state is when one Parliament has ultimate power over all of its constituent states, an example being the United Kingdom. This should not be confused with the most basic form of the unitary executive theory, in which a single individual has ultimate responsibility for the executive, or in other interpretations of that theory, where the executive is supreme.
Finally, constitutions may be supreme or subordinate. A supreme constitution is considered sovereign, and not subject to change from any superior laws. In the case of former colonies, their constitutions may be subordinate, having been drafted and introduced by an external sovereign power, and in theory able to be repealed by that power.
The process for amending constitutions is generally quite difficult, often requiring majority votes from all parliamentary houses. The public may also have a say in constitutional amendments if the issue is put to them in a referendum. Specific percentages in order for the amendment to succeed depend on the nation in question.
- Laurence H. Tribe: The Invisible Constitution, Oxford University Press. 2008