This article classifies the democratic party systems of the world by using the relative-size triangle, a bounded diagram that graphically represents information about seat distributions by party. In particular, the article solves identification problems stemming from the fact that party systems, as recurring patterns of party competition, involve systemic properties that are not reducible to the properties of individual party constellations. On this basis, 162 party systems that existed or continue to exist in the world’s democracies from 1792 to 2009 are assigned to the predominant party, two-party and multiparty types, each of the types being divided into two subcategories. A new measure of party system fragmentation, the systemic effective number of parties, complements the resulting classification with a methodologically consistent assessment of the degrees of multipartism. On this basis, the article arrives at several empirical conclusions regarding the geographical/chronological spread of different party systems and their relative longevity.