An analysis of the structure of parliaments in European countries shows that a wide range of options developed across the centuries. However, many of these patterns (among which tetracameralism, tricameralism, and qualified unicameralism) did not survive, despite their sometimes-remarkable historical interest. Currently, parliaments in Europe are either unicameral or bicameral: while unicameralism is the most common option, bicameralism is generally adopted in more populous countries and/or States with strong territorial autonomies. As a matter of fact, among varieties of bicameralism, the most common is characterized by a ‘territorial’ second chamber. Nevertheless, other types of bicameralism deserve attention too, not only to provide a comprehensive outline of the comparative scene, but also to find features that can define emerging trends. For this purpose, a classification of bicameralism will be outlined, mainly examining the patterns displayed by second chambers and the relationships between the two chambers. Combining this classification with the outcomes of the choice between unicameralism and bicameralism, some trends can be detected, although national experiences are so diverse that reliable norms are difficult to identify.
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