Abstract: In what he called "the liberal principle of legitimacy," John Rawls proposed that coercive exercises of political power can be justified to free and equal dissenters when "in accordance with a constitution (written or unwritten) the essentials of which all citizens, as reasonable and rational, can endorse . . . ." Does "unwritten constitution" there refer to empirical regularities of political practice (as opposed to normative rules and standards)? To norms that subsist only as custom but not as law? To norms that subsist as common law but not as code law? Which interpretation is best?