Article 41 concerns the case of two or more (but not all) parties to a multilateral treaty. The modification by certain parties of a treaty inter se, a particular form of "contracting out" from the original agreement, is well established in international law, the technique being employed, inter alia, to adjust a treaty to changing requirements or to ensure particular standards among some of the parties. Modification may concern the entire treaty or only certain provisions. The possibility of such a modification may be provided for by the treaty or the treaty shall not prohibit the modification. Modification shall not relate to a provision, derogation from which is incompatible with the effective execution of the object and purpose of the treaty as a whole. Article 41 is silent on the relationship between the inter se-agreement and the original treaty.