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An important, though oft neglected, distinction between multilateral treaty obligations separates obligations of the bilateral nature from those of the collective or erga omnes partes type. Multilateral obligations of the bilateral type can be reduced to a compilation of bilateral, state-to-state relations. They can be compared to contracts. Collective obligations, in contrast, cannot be divided into bilateral components. They are concluded in pursuit of a collective interest that transcends the individual interests of the contracting parties. The standard example of such obligations are those arising under a human rights treaty. In domestic law, collective obligations can be compared to criminal law statutes or even domestic constitutions. This essay examines the origins of the distinction between bilateral and collective obligations, as well as its major consequences, both in law of treaties and the law on state responsibility. On that basis, a wider typology of multilateral treaty obligations is suggested. In the exercise, obligations arising under the World Trade Organization are used as a case study. The argument is made that WTO obligations remain essentially of the bilateral type; they are not collective in nature.

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