CLEAVAGE POLITICS AND PARTY SYSTEM FORMATION This paper examines the impact of political legacies and social cleavage on party systems and how those systems in authoritarian and competitive authoritarian states are first structured. The initial purpose is to extend the debate on cleavage theory, apply it to new contexts, and show how social cleavage structures take shape. A secondary purpose, related to the first, is to reveal the early effect of social cleavage on party systems and party types. Using comparative analysis, the paper investigates the historical basis for understanding social cleavages and how they are expressed and gain saliency by interacting with party systems in the Middle East and North Africa (MENA), and Turkey. Specifically, it analyzes the cause and effect of post-independence political cleavages on early patterns of political mobilization and party systems, together with the influence of the latter on regime formation. The paper makes several points. First, where elites promoted state-centered nationalism, secular nationalist regimes emerged and were characterized by the dominant or one-party system. This created an ideological cleavage where a rival opposing force ‒ namely, Islamic nationalism ‒ generated several regime types, which also, like the secular elites, varied according to the level of modernization and their attitude to the West. This argument supports the importance of contextualizing cleavage politics in MENA. Second, this paper casts light on the nature of structural cleavages in MENA, conceptualizing those cleavages within its context advances our understanding of the origins and durability of the preponderant parties in the Middle East and their impact on regime type. Third, while traditional cleavages are a social construct, they can be politically constructed and manipulated by and imposed from above (a top-down ideology), particularly during regime transitions, and in responding to the political environment.