The law produced by the State is ordinarily described as municipal, national, domestic or internal law. The relationship between international law and municipal law can give rise to many practical problems, especially if there is a conflict between the two, or if a rule produced within one legal system is claimed to have relevance in another. The general rule of international law is that a State cannot plead a rule of, or a gap in, its own municipal law as a defence to a claim based on international law. States are required to perform their international obligations in good faith, but they are at liberty to decide on the modalities of such performance within their domestic legal systems. Rules for the recognition of customary international law in the internal sphere are either laid down in advance in the constitution or are gradually formulated by the national courts.