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\u27s 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 t...