An uncodified System is a type of constitution where the fundamental rules often take the form of customs, usage, precedent and a variety of statutes and legal instruments. An understanding of the constitution is obtained through reading commentary by the judiciary, government committees or legal experts. In such a constitutional system, all these elements may be recognized by courts, legislators and the bureaucracy as binding upon government and limiting its powers. Such a framework is sometimes imprecisely called an "unwritten constitution"; however, all the elements of an uncodified constitution are typically written down in a variety of official documents, though not codified in a single document.
An uncodified constitution has the advantages of elasticity, adaptability and resilience. A significant disadvantage, however, is that controversies may arise due to different understandings of the usages and customs which form the fundamental provisions of the constitution.
A new condition or situation of government may be resolved by precedent or passing legislation. Unlike a codified constitution, there are no special procedures for making a constitutional law and it will not be inherently superior to other legislation. A country with an uncodified constitution lacks a specific moment where the principles of its government were deliberately decided. Instead, these are allowed to evolve according to the political and social forces arising throughout its history.
When viewed as a whole system, the difference between a codified and uncodified constitution is one of degree. Any codified constitution will be overlaid with supplementary legislation and customary practice after a period of time.
The following states can be considered to have an uncodified constitution:
- Israel: the declaration of independence promised a constitution by 2 October 1948, but due to irreconcilable differences in the Knesset, no complete codified constitution has been written yet. There are several Basic Laws, however.
- New Zealand: see Constitution of New Zealand.
- Saudi Arabia: Saudi Arabia has no legally binding written constitution. In 1960, King Faisal declared the Quran to be the constitution. Although the Quran is the "Official Constitution of Saudi Arabia"; the Qu'ran is in fact the divine religious text of Islam and not a bespoke constitution for a certain sovereign state. See Constitution of Saudi Arabia. However, in 1992, the Basic Law of Saudi Arabia was adopted by royal decree.
- United Kingdom: there is no defining document that can be termed "the constitution". Because the political system evolved over time, rather than being changed suddenly in an event such as a revolution, it is continuously being defined by acts of Parliament and decisions of the Law Courts (see Constitution of the United Kingdom). The closest the UK has come to a constitutional code has been the Treaty of Union 1707, but this tends only to be subject to legal and academic scrutiny in Scotland, and has not received comparable attention in England and Wales. Due to the United Kingdom having an uncodified constitution it has meant that many acts have been added, e.g. The Freedom of Information Act 2000 and the Human Rights Act 1998.
- Canada: Although there is a Constitution Act  important aspects of the constitutional system are uncodified. The preamble to the Constitution of Canada declares that the constitution is to be "similar in principle to that of the United Kingdom" (which is uncodified.) This applies at the federal level and to the provinces, although each does have the power to modify or enact their own within their exclusive areas of responsibility. To date only British Columbia has done so (see Constitution of British Columbia), though the other provinces' roles and powers are spelled out in section 93 of the Constitution Act, 1867, and through amendments to it dealing with particular provinces such as the Manitoba Act and the Newfoundland Act. See Constitution of Canada.
- Constitution of the Roman Republic, made up of the Twelve Tables and other statutes.
- Hungary had an uncodified constitution prior to 1949.
- The Constitution of the Grand Principality of Finland was never codified. The Emperor of Russia, who also served from 1809 to 1917 as Grand Prince of Finland, never specifically recognized the Constitution as that of a separate and autonomous Finland, in spite of the fact that that Constitution largely dictated the relationship between Finland and the Russian Empire throughout the Russian era in Finland. By the late 19th century leading Finnish intellectuals - liberals and nationalists, and later, socialists as well - had come to consider Finland as a constitutional state in its own right in a mere real union with Russia. This notion clashed with emerging Russian nationalism and with Russian calls for a unitary state for Slavs only, which eventually came into conflict with Finnish separatism and constitutionalism in the form of "russification policies", which restricted Finland's extensive autonomy from 1899 onwards, excluding a brief interruption between 1905-1908, all the way to the February Revolution in 1917. The Russian Provisional Government of 1917 eventually recognized the Finnish constitution, and after the October Revolution the Bolshevik government of the RSFSR recognized Finland's declaration of independence on New Year's Eve 1917.
- Queensland before 2001. (See Constitution of Queensland.)
- Oman prior to 1996
- Libya between 1969 and 1975
- Johari, J.C (2006) New Comparative Government, Lotus Press, New Delhi, p167-169
- Prabir Kumar De (2011) Comparative Politics, Dorling Kindersley, p59
- Champion, Daryl (2003). The paradoxical kingdom: Saudi Arabia and the momentum of reform. p. 60. ISBN 978-1-85065-668-5.
- Robbers, Gerhard (2007). Encyclopedia of world constitutions, Volume 2. p. 791. ISBN 0-8160-6078-9.
- Constitution Act, retrieved 2012-03-25
- Ontario (Attorney General) v. OPSEU,  2 S.C.R. 2