A central feature of any constitution is the organization of the legislature. It may be a unicameral body with one chamber or a bicameral body with two chambers. Unicameral legislatures are typical in small countries with unitary systems of government (e.g., Denmark, Sweden, Finland, Israel, and New Zealand) or in very small countries (e.g., Andorra, Dominica, Luxembourg, Liechtenstein, Malta, and Tuvalu). Federal states, whether large or small, usually have bicameral legislatures, one house usually representing the main territorial subdivisions. The classic example is the Congress of the United States, which consists of a House of Representatives, with 435 members elected for two-year terms from single-member districts of approximately equal population, and a Senate, consisting of 2 persons from each state elected by the voters of that state. The fact that all states are represented equally in the Senate regardless of their size reflects the federal character of the American union. The U.S. Senate enjoys special powers not shared by the House of Representatives: it must ratify by a two-thirds majority vote the international treaties concluded by the president and must confirm the president’s appointments to the cabinet and to other important executive offices. The federal character of the Swiss constitution is likewise reflected in the makeup of the country’s national legislature, which is bicameral. One house, the National Council, consists of 200 members apportioned among the cantons according to population; the other house, the Council of States, consists of 46 members elected from the cantons by direct vote.

Argentina, Brazil, Mexico, and Russia possess federal systems that mirror the U.S. model of equal representation for each subnational government in the upper chamber (since the mid-1990s one-fourth of Mexican senators have been elected in a single national district on the basis of the proportion of votes their political parties receive). In some federal systems representation of regions in the upper house is not equal. In Germany, for example, states are allocated three to six seats in the upper house (the Bundesrat), depending on population. In federal Austria each state is guaranteed at least three seats in the Bundesrat. In federations in which there is no guaranteed overrepresentation of smaller regions, a crucial principle of federalism is violated: the protection of regional sovereignty against a central government, backed by a national majority, that may seek to erode regional autonomy. An example of this case is Canada, where the upper house (the Senate) is an appointed body that is not constitutionally required to represent the provinces, though in practice senators are appointed (for life terms) to ensure regional balance. Although Micronesia and Venezuela are both federal states, each has a unicameral legislature.

A unitary system of government does not necessarily imply unicameralism. In fact, the legislatures of most countries with unitary systems are bicameral, though one chamber is usually more powerful than the other. The United Kingdom, for example, has a unitary system with a bicameral legislature, which consists of the House of Lords and the House of Commons. The Commons has become by far the more powerful of the two chambers, and the cabinet is politically responsible only to it. The Lords has no control over finances and only a modest suspensory veto with respect to other legislation (it may delay the implementation of legislation but not kill it). A veto by the Lords can be overcome in the Commons by a second vote at an early date. The parliaments of Italy, Japan, and France also are bicameral, though none of those countries has a federal form of government. Although in the United States all 50 states except Nebraska have bicameral legislatures, their governmental systems are unitary. In the 49 U.S. states with bicameral legislatures, the two houses have equal legislative authority, but the so-called upper houses—usually called senates—have the special function of confirming the governors’ appointments.