In cross-national research on party systems, the empirical units of analysis are often assumed to be self-evident, which can be conducive to misleading research results. This problem is particularly important with regard to party system classification, for which a methodologically rigorous approach to the units of analysis is needed. This article proposes a set of operational criteria for identifying elements that qualify for inclusion within the universe of democratic party systems among individual election outcomes and country-specific sequences of elections. On this basis, I introduce additional criteria for distinguishing between party systems and party non-systems, and among party systems evolving within the same nation-state settings. By applying these criteria to a set of 1502 national legislative elections held in the world’s democracies from 1792 to 2009, the article identifies 162 units that can be entered into a classification of the world’s democratic party systems and 21 party non-systems.