Bicameral legislatures are most commonly found in countries with a federal system of government. In a federal system, power is divided between a central government and regional governments, such as states or provinces.
The two chambers of a bicameral legislature often have different functions and may represent different constituencies. For example, in the United States, the Senate represents the interests of the states, while the House of Representatives represents the interests of the people. This allows for a balance of power and helps to ensure that different perspectives are taken into account when making laws.
Bicameral legislatures can also be found in some unitary systems, where power is concentrated in a central government but regional governments still have some authority. However, this is less common than in federal systems. Bicameral legislatures are not typically found in confederal systems, where power is held primarily by regional governments.
Finally, whether a system is rigid or flexible is not directly related to the presence of a bicameral legislature. A rigid system generally refers to a constitution that is difficult to change, while a flexible system allows for easier changes to the constitution.