Scholarship on the rise of political parties mostly focuses on the electoral arena. From this perspective, the legislative arena is determined by party system properties. This article aims to show that the causal arrow points in the other direction as originally suggested by Duverger. Two mechanisms of legislative organization arguably allow for the ascendancy of political parties: control over the legislative agenda or powerful committees. A comparative analysis of thirty-six procedural reforms in the Swedish and French legislatures during the 1866-1958 period suggests that, all else being equal, a quest for procedural efficiency originating from the dynamics of legislative democracy led to the creation of powerful committees. Only if anti-system obstruction of legislation posed a threat to the survival of legislative democracy, individual legislators became willing to unilaterally surrender inherited powers to control the plenary agenda to party leaders. Since both the empowerment of committees and the centralization of agenda control occurred independent of the properties of party systems, parties are the product rather than the origin of legislative democracy.