This Article argues that, contrary to the consensus of economists and many legal scholars, the norm of "horizontal equity" in taxation has independent meaning as a default rule in favor of existing arrangements. Although it has long been said, and widely thought, that tax should be fair in its dealings with individuals who are situated similarly to one another, no one has been able to say convincingly just what that fairness comprises. As a result, the learned referees in the last major dispute over the significance of horizontal equity judged that fairness's critic had decidedly won the day. Since then, there have been ever more critics, but no cogent, comprehensive defense. My defense is both theoretical and practical. First, I argue that horizontal equity is a special aspect of the revenue function in taxation. Because it enshrines the status quo before enactment of a new tax law, horizontal equity can be reconceived as a commitment by the authors of tax legislation to honor the past and future policy choices of others, with whom they are jointly engaged in a project of deliberative democracy. Alternately, horizontal equity may bejustfied by welfare gains from a shared agreement to leave certain controversial questions of distributive justice undecided during the revenue-raisingp rocess. Both oft hese rationalesl eave open-indeed,t hey clear the air for-arguments about the ultimate ends law, and tax law in particular, should serve in society.