This research note traces the evolution of Sartori's theoretical reflection on the party system, primarily by describing the relationship between “format and mechanics” to assess its explanatory power. The main conclusion of the analysis is that Sartori's framework has all the elements of an empirical party system theory. A systemic theory of party relations does not have to explain party behaviour but only the combined effect, in terms of the system's mechanics, of all party actions. Thus, the theory states that the number of parties and their positioning in the unidimensional competitive space cause the system's mechanics. The dependent variable is ordinal, about the quantitative and qualitative distribution of power among the parties. This detracts from the parsimony of the theory and requires other independent or at least intervening variables besides the number of parties: ideological distance and direction of competition. This difference notwithstanding, we can still accept Sartori's framework as a general party system theory. Appropriate mid or lower-range theories could supplement the general one in explaining sub-systemic phenomena. Irrespective of whether we call it a classification, a framework or a theory, Sartori's contribution remains a fundamental milestone in the study of parties and party systems.