A large body of scholarship places considerable weight on the role that national constitutions play in promoting order and development. To what degree are these institutions fixed "rules of the game" or fluid outcomes, responsive to changes in underlying primitives? In this paper, we develop a dynamic measure of constitutional similarity to show that the typical national constitution is hardly fixed. We find, in contrast, evidence of a large degree of fluidity and change: over one-third of all variation in constitution writing is driven by within-country changes. We then investigate broad trends in constitution writing and find that across the twentieth century there has been a convergence in constitutional forms of government. Finally, we provide evidence that this trend has been toward documents that contain diffuse centers of power and numerous well-defined, positive, rights.