We present a framework for understanding and interpreting reciprocity and nondiscrimination, the two principles that are the pillars of the multilateral trading system as embodied in GATT and its successor, the WTO. We show that GATT's principle of reciprocity serves to neutralize the world-price effects of a country's trade policy decisions, and hence can deliver efficient trade-policy outcomes for its member governments provided that the externalities associated with trade intervention travel through world prices. We then establish that externalities indeed travel in this way if and only if tariffs also conform to the principle of non-discrimination (MFN). In this way, the principles of reciprocity and non-discrimination can work in conce...