This article aims to describe how the two-chamber solution developed and the reasons for its establishment, to shed light on the historical roots of this institutional framework. Three phases are identified: the pre-constituent debate, the ideas contained in the documents of the Ministry for the Constituent Assembly on the structure of the legislative chambers, and finally the work of the Constituent Assembly itself. The institutional solution of equal and perfect bicameralism, unique in the European constitutional scene, is not particularly systematic. The attempt to provide a rational foundation for bicameralism is difficult: indeed, it is a two-chamber parliament, but it is unicameral in functional terms because the chambers are undifferentiated; ultimately, it was reached because of the cross-vetoes opposed by the parties. Indeed, each mutually feared each other’s victory and deep divisions ran between the political forces.