The law governing the admission of reservations to treaties has evolved over the past sixty years, as is demonstrated by the Vienna Convention on the Law of Treaties, which incorporates a regime on reservations that represents a significant change in the law as it was generally understood in the first part of this century. Whereas it was once widely considered that the unanimous consent of the other parties to a treaty was required to admit a reservation, the Vienna Convention implemented a new system in which this condition is generally not necessary. The regime on reservations is contained in section 2 of the Convention (Articles 19-23), which, together with the definitional article (Article 2(1)(d)), determines what constitutes a reservation, the criteria it must meet to be permissible, and the effect it will have, both in the event that it is accepted by other contracting states and in the more problematic event that it is not.