Bicameralism

A bicameral legislature is one in which the legislators are divided into two separate assemblies, chambers or houses. Bicameralism is distinguished from unicameralism, in which all of the members deliberate and vote as a single group, and from some legislatures which have three or more separate assemblies, chambers or houses. As of 2015, somewhat less than half of the world's national legislatures are bicameral.

Often, the members of the two chambers are elected or selected using different methods, which vary from country to country. Enactment of primary legislation often requires a concurrent majority, the approval of a majority of members in each of the chambers of the legislature. However, in many Westminster system parliaments, the house to which the executive is responsible can overrule the other house.

Bicameralism is an essential and defining feature of the classical notion of mixed government.

One may distinguish perfect bicameralism and imperfect bicameralism.

Although the ideas on which bicameralism are based can be traced back to the theories developed in ancient Sumer and later ancient Greece, ancient India, and Rome, recognizable bicameral institutions first arose in Medieval Europe where it was associated with separate representation of different estates of the realm. For example, one house would represent the aristocracy, and the other would represent the commoners.

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