Law and custom supplement and complement each other. Activities that were once performed earlier are now consciously formulated. According to Maine there is a necessity for law to adjust itself to social necessities and social opinion. When a law expresses the moral consensus of the society it will be effectively enforced. If it is not backed by firm moral consensus effective enforcement is less likely. A law that does not give official sanction to customs looses a certain force of sentiment which customs have behind them and which helps in obedience. According to Edmund Burke manners are of more importance than laws. Upon them in great measure the laws depend. Customs consolidate law and facilitate its practice. If the law is not aided by customs it cannot succeed. Certainly a law cannot succeed permanently if it is opposed by deep-seated customary attitudes. Custom is an important source of law. The passing of laws against certain practices can be effective when the mores underlying those practices are in the process of disintegration and a considerable number of people no longer hold to them because then it is mostly a matter of forcing the reluctant ones to accept the new legal way. Law supplements custom. Today law is being used to change obsolete customs like untouchability, dowry system, child marriage etc.
In the primitive societies custom well served to regulate the conduct of life but in modern urban-industrial societies customs become blurred and are challenged by newly emerging loyalties and interests. Today law takes a large part in total social control. There are several reasons for it. Custom lacks an agency of authoritative jurisdiction due to which the interests of the community do not remain fully secure. Law with special agency of enforcement is required if interests are to be pursued in peace. Customs cannot adapt itself readily to changing conditions. Being fixed and permanent customs change very slowly. Social necessities are always in advance of custom. Therefore to meet the social necessities and for quick adaptation to changing conditions another kind of code is required a code which does not slowly evolve but one which is made expressly for the situation. Since there are different customs of different groups therefore to have a single and uniform rule of behavior where it is desirable, it is necessary to supplement custom by law. Modern developments in the economic field have occasioned enormous addition to law everywhere.