The term «amendment» refers to the formal alteration of treaty provisions affecting all the parties to the particular agreement. Such alterations must be effected with the same formalities that attended the original formation of the treaty. Many multilateral treaties lay down specific requirements to be satisfied for amendments to be adopted. In the absence of such provisions, amendments require the consent of all the parties.
Article 39 VCLT: ‘A treaty may be amended by agreement between the parties’.
1st: The particular procedure of the concerning treaty.
2nd: if the concerning treaty has not provided any rule, article 40 VCLT establishes:
- Amendments must be notified to the rest of the States. Any State could negotiate and conclude that amendment.
- ‘The amending agreement does not bind any State already a party to the treaty which does not become a party to the amending agreement’.
Modification: a number of parties agree to modify the effects of the treaty among themselves. The term «modification» refers to the variation of certain treaty provisions only as between particular parties of a treaty, while in their relation to the other parties the original treaty provisions remain applicable. If the treaty is silent on modifications, they are allowed only if the modifications do not affect the rights or obligations of the other parties to the treaty and do not contravene the object and the purpose of the treaty.
In accordance with article 41 VCLT, only when it is allowed expressely, it is not prohibited expressely, it does not affect ‘the enjoyment by the other parties of their rights under the treaty or the performance of their obligations’ or it is not incompatible ‘with the effective execution of the object and purpose of the treaty as a whole’.