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This article addresses long-standing challenges in conceptualizing and measuring de/centralization in unitary and federal systems. It first outlines a novel conceptualization of de/centralization and its relation to the unitary/federal distinction and a/symmetry, from which it derives static and dynamic typologies of state structures. Building on a critical review of the Regional Authority Index, the article develops a scheme for measuring de/centralization and applies it to map state structures in Western Europe from 1950 to 2015. This mapping exercise shows that the proposed scheme yields more accurate measures of de/centralization than existing indices and that de/centralization differs in kind, rather than merely in degree, between unitary and federal states. By enabling a more effective classification of cases in comparative analysis, the article offers a tool on which theoretical and empirical advances in …