In contrast to unicameralism, and bicameralism, multicameralism is the condition in which a legislature is divided into more than two deliberative assemblies, which are commonly called "chambers" or "houses".^[1][2] This usually includes tricameralism with three chambers, but can also describe a system with any amount more. The word "multicameral" can also relate in other ways to its literal meaning of "many chambered" with use in science or biology.

Approximately half of the world's sovereign states are unicameral, and newer democracies and more recent constitutions are more often unicameral than not. More specifically many countries have switched to unicameralism whereas the opposite is rare. Nevertheless, many current parliaments and congresses still have a multicameral (usually bicameral) structure, which some claim provides multiple perspectives and a form of separation of powers within the legislature.^{[citation needed]}

Many societies in Medieval Europe had quasi-legislative assemblies in the form of the Estates of the Realm, typified by those of France. Typically, this body had three chambers representing the three grand divisions of society; the clergy, nobles, and commoners; however, this was not universally the rule; Medieval Scandinavian deliberative assemblies traditionally had four estates: the nobility, the clergy, the burghers, and the peasants. The Swedish and Finnish Riksdag of the Estates survived the longest of these bodies, having four separate legislative houses. Sweden abandoned its four-chamber parliament in 1866, transitioning to a bicameral Riksdag for more than a century before moving to today’s unicameral assembly in 1974 (see History of the Riksdag).

When the Grand Duchy of Finland was seized by Russia from Sweden, the four-chambered Diet of Finland, of identical structure to the Swedish Riksdag of the Estates, was established. It continued to legislate for Finland until 1906, being the only ancient legislature to survive to the 20th century while maintaining the traditional estates. In that year, the Diet’s four ancient chambers were disbanded and replaced by the modern unicameral Parliament of Finland.

The Parliament of England developed in the opposite direction, merging the two aristocratic estates into the House of Lords, the archetypal upper house,leaving the House of Commons as the elective lower house; in time, the English and later British parliaments became the standard model on which the modern bicameral legislature is based.

Of the ancient assemblies in Europe, only Finland’s survived to see the 20th century. As the armies of Revolutionary France conquered much of Europe in the name of liberalism and popular sovereignty, most countries’ newly established or re-established legislative assemblies were structured after either the (originally) unicameral French National Assembly or the bicameral British Parliament. Since the 19th century, tricameral legislatures have been a rare constitutional curiosity, with the overwhelming majority of assemblies having one or two chambers.

The Federal Assembly of Yugoslavia originally had five chambers. After Yugoslavia adopted a new constitution in 1963, its legislature was restructured into four chambers each representing the various sectors of Yugoslav society with an additional chamber representing the general population.^[3][4] The Federal Assembly was the only legislature anywhere with five chambers, and a constitutional amendment added a sixth component described as either a chamber or sub-chamber.^[5][6][7][8] Yugoslavia adopted yet another constitution in 1974, abolishing the Federal Assembly and replacing it with a bicameral legislature.^[9]

Perhaps the best-known multicameral assembly in modern times is the Tricameral Parliament of the waning days of Apartheid South Africa; established in an effort to stabilize the collapsing Apartheid system, it was intended to give limited representation to the country’s Cape Coloured and Indian populations to stabilize white-minority rule. The assembly failed to stem calls for universal suffrage, and was tremendously unpopular with the non-white population. When apartheid was abolished, the Tricameral Parliament disappeared with it, replaced with today’s bicameral assembly.

Proponents of multicameral legislatures hold that multiple legislative chambers offer the opportunity to re-debate and correct errors in either chamber in parallel, and in some cases to introduce legislation in either chamber.^{[citation needed]} Advocates of multicameralism also contend that multiple legislative chambers are (best) able to represent the various important sectors of society (such as culturally or linguistically distinct, geographically different or similarly interested populations that comprise a country - i.e. the various states of the United States of America or provinces of Canada, each with their own geographical borders, subcultures, interests and even languages i.e. English, French, Spanish), which may not be able to be adequately represented by a singular legislative body. Supporters of multicameralism also posit that a critical weakness of a unicameral system can be a potential lack of restraint on the majority (mob rule) and incompatibility with the separation of powers between the legislative and executive branches of government, particularly noticeable in parliamentary systems where the leaders of the parliamentary majority also dominate the executive.

• Federalism
• Polycentric law