Customary law and conventional law are primary sources of international law. Customary international law results when states follow certain practices generally and consistently out of a sense of legal obligation. Recently the customary law was codified in the Vienna Convention on the Law of Treaties . Conventional international law derives from international agreements and may take any form that the contracting parties agree upon. Agreements may be made in respect to any matter except to the extent that the agreement conflicts with the rules of international law incorporating basic standards of international conduct or the obligations of a member state under the Charter of the United Nations.
International agreements create law for the parties of the agreement.
They may also lead to the creation of customary international law when they are intended for adherence generally and are in fact widely accepted. Customary law and law made by by international agreement have equal authority as international law. Parties may assign higher priority to one of the sources by agreement. However, some rules of international law are recognized by international community as peremptory, permitting no derogation. Such rules can be changed or modified only by a subsequent peremptory norm of international law.