This essay studies theory of lex mercatoria which is based on the assumption of existence of a third legal system along with the national and international legal systems regulating international commercial relations. For this purpose, the essay discusses the backgrounds of creation and emergence of the theory, its narrow and broad meaning, elements of which and the extent that it has been recognized at the international and national levels i.e. by national legal systems including Iranian legal system. For the sake of impartiality, the last part has been devoted to criticisms raised regarding the existence and sources of lex mercatoria, the suggested alternative and a final evaluation.
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