This study uses empirical evidence from a nearly comprehensive set of electoral democracies that use single-member plurality systems or mixed electoral systems with single-member plurality components in order to establish empirically the impact of territorial patterns of electoral support upon vote-to-seat conversion. The analysis, employing individual parties as units of analysis and multiple linear regression with cluster-robust standard errors as its main methodological tool, confirms the hypothesis that single-member plurality rules give a representational bonus to parties with low levels of nationalization. This effect is contingent upon absolute and relative party size, so that very large parties, and particularly frontrunners in party competition, receive an advantage irrespective of their nationalization. Mixed electoral rules further facilitate their advantage, while the gains of very small parties tend to be enhanced by large assembly size. These findings contribute to a better understanding of the dynamics of party system development under Duvergerian conditions.