Duverger classified systems primarily by the number of parties: two-party, multi-party, and one-party. Sartori rejected this simplicity, arguing that the number alone tells us nothing about how the system actually works. He introduced the crucial distinction between moderate pluralism (3-5 parties with low ideological distance, centripetal competition) and polarized pluralism (5+ parties with high ideological distance, centrifugal competition, anti-system parties). This captured the crucial difference between systems like post-war Germany and Weimar Germany—both had multiple parties, but their dynamics were radically different.

Sartori argued that not all parties in a system matter equally. A party is 'relevant' if it has either coalition potential (it can feasibly enter governing coalitions) or blackmail potential (its existence forces other parties to respond to it, shifting the entire political spectrum). This criterion is essential because it excludes fringe parties from systemic analysis while capturing the influence of extremist parties that may never govern but still reshape politics. It explains why the Communist Party in post-war Italy was systemically relevant despite never being in government.

Sartori identified polarized pluralism as a system with more than five parties, high ideological distance, anti-system parties on the extremes, and centrifugal competition. His eight characteristics include: presence of anti-system parties, bilateral opposition (opposed on both left and right), center filling (a central party occupies the middle), ideological distance, centrifugal drives, two-way competition, irresponsible opposition, and permanent elections that become plebiscites. This concept became foundational for analyzing unstable democracies like Weimar Germany, Chile under Allende, and contemporary cases of extreme polarization.

In a predominant-party system, a single party consistently wins absolute majorities (e.g., Sweden's Social Democrats pre-2006), but the opposition remains free and competitive elections continue. In a dominant-party system, one party dominates indefinitely due to structural advantages or suppression, making alternation effectively impossible (e.g., PRI in Mexico, the Liberal Democratic Party in Japan). Sartori's insistence on this distinction was crucial because it prevents conflating democratic dominance with authoritarian hegemony, preserving the analytical distinction between democracy and dictatorship.

Sartori was a methodologist of concepts. His work on conceptual stretching—the danger of applying terms so broadly that they lose meaning—and the 'ladder of abstraction' has influenced every subfield of political science. He argued that scholars must clearly define concepts, maintain conceptual coherence across cases, and avoid the 'category fallacy' of imposing Western categories on non-Western contexts. His *Comparative Constitutional Engineering* (1994) applied this rigor to comparing presidential and parliamentary systems, electoral laws, and constitutional design, making conceptual clarity the hallmark of his entire intellectual project.

Sartori's expectation that polarized pluralism is inherently unstable and tends toward democratic breakdown has been challenged by cases like contemporary Italy or India, where high fragmentation and polarization have persisted without collapse. His model underestimated the role of institutions like strong presidentialism or federalism in managing centrifugal forces. Additionally, his work offered limited tools for analyzing the rise of anti-establishment populist parties in stable democracies (e.g., Italy's Five Star Movement), which fit awkwardly into his bipolar left-right framework. Critics argue his typology is too static and Eurocentric.

A widespread error is treating polarized pluralism as simply 'many parties.' Sartori explicitly warned that the number of parties alone is insufficient. Polarized pluralism requires both high ideological distance and the presence of anti-system parties—without these, multiple parties produce moderate pluralism, which is stable. This misconception has led to mislabeling countries like Canada (with multiple regional parties but low ideological distance) as polarized pluralist. Sartori's emphasis on ideology and systemic interaction, not just arithmetic, remains his enduring methodological lesson.