Looking more concretely at customary law, it means law that, through practices repeated in social life, has given the general public a legal conviction or awareness and has thus acquired legal value even though it is not codified. Customary law must have legitimacy and reasonableness that do not violate the entire legal order. Modern states did not recognize customary law. The requirements for customary law to arise are as follows: first, the custom must exist as a repeated practice, and its content must include legal value carrying the conviction or awareness of being a legal norm. In addition, the custom must not be contrary to public order and good morals, and it must not violate the entire legal order or must in principle concern matters not provided for in statutes.

Case law is law that takes on the character of a legal norm without going through codification where, as judgments to the same effect are repeated on a certain legal issue, their direction has become largely settled. Case law is a special form of customary law and differs from general customary law in being formed in the courts. Countries of the Anglo-American legal tradition adopt the principle of binding precedent, under which, once a higher court has rendered a judgment on a legal issue, that court or lower courts cannot afterward reach a result different from the earlier judgment when dealing with the same legal issue. Since countries of the continental legal tradition adopt the principle of written law, the principle is that courts are not bound by the judgments of courts of the same or higher level. Therefore, case law also has the limitation of being no more than a secondary source of law.

What written law and unwritten law have in common is that both are sources of law. A source of law is generally understood as the basis for recognizing law. Sources of law include, regardless of the form in which they appear, all guidelines and standards for conduct that fix the purposes or standards of administrative acts, prescribe models of conduct and decide judicial determinations of disputes. That is why not only written law but also unwritten law belongs to the sources of law.

On the other hand, the differences between the two are as follows. Article 1 of the Civil Act recognizes the superiority of written law over customary law, and customary law is in principle recognized as having supplementary effect only for matters on which written law has no provision. In particular, where written law recognizes the existence of customary law different from the content of its provisions, customary law is recognized as having the effect of amending or repealing the written law.

For reference, the material writes that if there is customary law different from written law, it is recognized as having the effect of changing the written law, but the case law holds that customary law has only supplementary effect to the extent it does not conflict with statutes. However, Article 1 of the Commercial Act provides that, for commercial matters on which the Commercial Act has no provision, commercial customary law applies first and then the Civil Act, so there is an exception in which customary law takes precedence over the Civil Act. As an example of customary law losing effect due to social change, also look at the 2005 Supreme Court en banc decision on the custom of regarding only adult men as members of a clan association.