Abstract
Custom in the form of tradition and practice and customary habitude among people is the lifeblood of any legal entity and is considered as one of those subjects with unmatched impact on conformity of certain jurisprudence decrees to demands of time and place. Indeed, the more a legal system is affected by custom, the more consolidated and stronger it will be. Islamic legal system accrediting local customs and legislator‘s referral to them has been able to maintain its live and dynamic nature. This paper was performed to explain the position and effects of the custom in Islamic legal system noting its role in Iranian legal system. Therefore, after expressing conception of the custom from viewpoint of Shiite and Sunni jurisconsults as well as its idiomatic definitions, we practiced to deal its effects on jurisprudence and law and Iranian legal system through expressing features of custom as the main source and base in jurisprudence and law, its position deducing religious orders and its impacts on Iranian legal system.
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