An electoral system is an essential component of representative democracy. It translates preferencesof citizens to a legislative body and inevitably distorts preferences, voicing some more loudly than others.Theorizing and empirically analyzing how the electoral system tilts the playing ground is the aim of thisstudy.The number of seats allotted to an electoral district—the district magnitude (DM)—is perhaps themost important component defining an electoral system. It is long established that DM affects key featuresof the political landscape in a country, such as representation, the number of parties, the type of government(single- or multi-party coalition), parties’ strategy, voters’ consideration, and even redistribution policy.Most democracies, however, have districts of many different magnitudes, and the range often reaches thirtyseats gap between the smallest and largest districts in a country. Districts in Portugal, for instance, varybetween two and forty-eight seats, and in Switzerland between one and thirty-five. The voluminousliterature on electoral districts uniformly sidesteps this heterogeneity, focusing instead on a single middledistrict per country.The proposed study is the first large-scale study that theorizes about and empirically analyzes theeffects of within-country district structure. I address questions such as: how does district heterogeneityshape representation at the national level? How does it affect the party system? And how does it affect partycoordination?In the first part of the study I will theorize about various aspects of district heterogeneity in a country(e.g., skewness, effective number of magnitudes). I will gain deep understanding for district distributionsand develop politically-relevant measures of heterogeneity. Drawing on insights from the theoretical part,the second part will empirically examine how district heterogeneity affects the political landscape, and inparticular representation, party system, and party coordination. This part relies on extensive district- andnational-level data collection and data analysis in OECD countries as well as in-depth case analysis.Analyzing the effect of district heterogeneity on representation, party systems, and partycoordination will open new avenues of research about design of electoral systems.