What makes a constitution legitimate? Models grounded in consent, right procedure, or necessary and sufficient justice conditions capture powerful intuitions, but face equally powerful problems: These models generate paradoxes and infinite regress, and their static character ignores legitimacy’s dynamism. Moreover, debates around constitutional interpretation – originalism, living tree, or common good oriented - demonstrate the permanent space between a (constitutional) rule and its application. These debates leave mysterious how legitimacy, once in a constitution, ever gets out. But these issues resolve if we understand legitimacy as something functional, not substantive. Like a currency, I suggest, it can be drawn from diverse (normative and symbolic) sources, banked (in constitutions), and later withdrawn and spent (on political endeavours). This model honours normative intuitions, while escaping puzzles and paradoxes. Moreover, since a constitution’s legitimacy ‘holdings’ can fluctuate with political skill and circumstance, this model capture’s legitimacy’s dynamism. Such a functional model bridges the empirical and normative study of legitimacy, and it may deepen empirical understanding of normativity’s role in regime stability and constitutional change.