392. With reference to their legal effects, there are three kinds of customs: (a) customs according to the law, which are those that confirm by use an existing law; in this way custom interprets law (see 386); (b) customs beside the law, which are those that introduce a new obligation that is not prescribed by any written law; in this way custom establishes law; (c) customs contrary to law, which are those that remove the obligation of a previous law; in this way custom repeals, at least in part, the law to which it is opposed. 393. Custom has not the power to establish or repeal a law, unless it possesses the requisites of law itself (see 285). Hence arise the following conditions: (a) Since the exercise of the legislative power requires freedom, customs do not possess legal force unless they have been practised freely. Hence, a custom that has been established by force does not suffice; (b) Since laws can be made only for perfect societies, customs have not the force of law, unless they are practised by a perfect society, or by a majority of its members who are representative. Hence, a custom observed by a family or by a minority of the voters in a body that has its own jurisprudence has not the status of law; (c) Since laws must proceed from competent authority, customs do not make or unmake law, unless they have the approval of the ruling power. In a society where the legislative function rests with the people (e.g., in the ancient democracy of Athens), the fact that they follow a custom with the purpose of enacting it into law or of using it against an existing law is sufficient approval. But if the supreme power is not with the multitude, their customs do not obtain the force of legislative acts, unless approved by the constituted authority; (d) Since law needs to be promulgated, a custom, to have the effect of law, must be practised by public acts through which it becomes known to the people as a whole. 394. Customs that have the other requisite conditions begin to be obligatory or derogatory as soon as the approval of competent authority is had. (a) If the approval is given expressly, the custom has the force of law at once; (b) if it is given tacitly, inasmuch as the lawgiver, knowing the custom and being under no restraint, does not disapprove, the custom has the force of law as soon as tacit consent is recognized by the learned and prudent; (c) if it is given by the law itself, which explicitly accepts reasonable customs, the custom has the force of law when it has lasted for ten years, or other length of time prescribed. 395. If the superior disapproves of a custom or maintains diplomatic silence for fear of greater evils, his consent is withheld, and the custom cannot be deemed as of legal force.