We provide a justification for political liberalism’s Reciprocity Principle, which states that political decisions must be justified exclusively on the basis of considerations that all reasonable citizens can reasonably be expected to accept. The standard argument for the Reciprocity Principle grounds it in a requirement of respect for persons. We argue for a different, but compatible, justification: the Reciprocity Principle is justified because it makes possible a desirable kind of political community. The general endorsement of the Reciprocity Principle, we will argue, helps realize joint political rule and relationships of civic friendship. The main obstacle to the realization of these values is the presence of reasonable disagreement about religious, moral, and philosophical issues characteristic of liberal societies. We show the Reciprocity Principle helps to overcome this obstacle.