This chapter develops models of what I call spatio-political structure, rooted in the differentiation of centers and peripheries. Distinguishing between a) the number of top-level political centers, b) the homogeneity or heterogeneity of centers, peripheries, and their relations, and c) the relative autonomy of centers and peripheries, I identify three principal types of political systems (both of which have “international” and “national” forms): systems of single-level governance (e.g., states systems); systems of single-center governance (e.g., empires); systems of multilevel multiactor governance (e.g., medieval Europe). In IR’s standard (Waltzian) structural framing, pre-defined units (individuals and states), on (three) pre-defined levels, combine into pre-defined types of (national and international) political systems. I instead treat as empirical questions the types of polities that exist within a space, their distribution and relations, and the resulting kinds of systems. The chapter concludes with a novel, heterarchic depiction of the turn-of-the-twentieth-century Eurocentric international system.