What explains the development of legislative party voting unity? Evidence from the United States and Britain indicate that partisan sorting, cohort replacement effects, electoral incentives, and agenda control contributed to enhancing party cohesion during the nineteenth and twentieth centuries. Here, these mechanisms are evaluated by analysing a dataset containing all the recorded votes from the Canadian House of Commons, 1867-2011. Overall, we find that partisan sorting and the government's ability to control the agenda are central to the consolidation of parties over time. Our results underscore the need to integrate institutional rules and legislative agendas into models of parliamentary voting behaviour and suggest that strict party discipline can lead to the development of a multi-party system in the legislative arena.The emergence of permanently organized and disciplined political parties represents one of the most important developments in the history of modern parliaments. 1 While there is a vast literature on the influence of parties in the legislative arena today, we lack a clear understanding of how party org1anizations transform over time. Scholars generally agree that as the influence of representative assemblies increased during the nineteenth century, a number of countries began experiencing major political changes that prompted lawmakers to modify their behaviour in the legislature. As a result, parties became increasingly unified, primarily in response to the extension of suffrage and the modernization of the legislative process. 2 Although previous research appears to suggest a direct relationship between parliamentary organization and electoral politics, we find a wide range of competing theories to explain the emergence of party cohesion in the legislative arena. 3 Thus far, scholars have claimed that several different factors, such as the centralization of the leadership structure or the changing ideological preferences of members (either through replacement or socialization), have contributed to an increased number of partisan votes in the British Parliament and American Congress throughout the nineteenth century. 4 And while this trend has been observed in many other established democracies since then, empirical studies of these older cases are scarce and