Both treaty law and customary international law are sources of international law. Treaties, such as the four Geneva Conventions of 1949, are written conventions in which states formally establish certain rules. Treaties bind only those States that have expressed their consent to be bound by them, usually by ratification. The modern codification of civil law evolved from the tradition of medieval custumals, collections of local customary law that developed in a particular property or district jurisdiction and were slowly composed mainly from case law and later written by local jurists. Custamal became law when they became the undisputed rule governing certain rights, claims and obligations between members of a community.  Some examples are De Legibus and Consuetudinibus Angliae de Bracton for England, the Coutume de Paris for the city of Paris, the Sachsenspiegel for northern Germany and the many Fueros of Spain. These can be subdivided into legal and treaty practices. In general, the purpose of international law is to regulate relations between States, and it is therefore binding on States. This also applies to international humanitarian law, whether contractual or customary, as it governs armed conflicts between States. In the Scandinavian countries, customary law continues to exist and has a great influence.
A peculiarity of international humanitarian law, however, is that some of its rules govern armed conflicts between a State and an armed opposition group, or between such groups. The rules governing these conflicts apply to all parties, whether a State or an armed opposition group. Analysis of State practice shows that many rules of customary humanitarian law applicable in non-international armed conflicts are binding on both States and armed opposition groups. As this school indicates, custom contains its own legitimacy, since it would not exist at all, unless certain deep needs of the general population or a local character of social needs give it validity. Any custom that implicitly or explicitly advocates or calls for violence „cannot be considered a custom. In international law, customary law refers to international law, or legal norms that have evolved over time through customary exchanges between states, whether on the basis of diplomacy or aggression. Essentially, it is believed that States have a legal obligation to conduct their affairs in accordance with the accepted conduct of the past. These practices may also change as a result of the acceptance or rejection of certain acts by States. Some principles of customary law have acquired the force of peremptory norms that can only be violated or modified by a norm of comparable force. These norms are supposed to derive their strength from general acceptance, such as prohibitions of genocide and slavery. Customary international law differs from treaty law, which consists of express agreements between nations to enter into obligations.
However, many treaties are attempts to codify pre-existing customary law. According to Carter, „The habit is only effective when it is universal or nearly universal. In the absence of unanimity, the habit becomes powerless, or rather does not exist. Whatever the public policy of the State in which the custom operates must be respected. For Salmond, a valid custom has absolute legal authority, which is the force of the law itself. It divides customs into two parts: In India, many customs are legally accepted. For example, Hindu wedding ceremonies are recognized by the Hindu Marriage Act. According to Salmond, the legal tariff has a legal obligation in itself or proprio vigore. It further divides legal practices into general and local customs, which have already been discussed. According to Hund, the second form of scepticism about rules states that although a community may have rules, these rules are not reached „deductively,” that is, they are not only created by legal/moral considerations, but are motivated by the personal/political motivations of those who create them.
The scope of such influence is created by the vague and indefinite nature of customary law, which Hund argues gives customary legislators (often through traditional „judicial proceedings”) broad discretion in its application. However, Hund argues that the fact that rules are sometimes set on an ad hoc basis does not mean that it defines the system. If you need a perfect system in which laws are created only deductively, then a system without rules remains. For dogs, this may not be the case, and an explanation of this type of legislative process can be found in Hart`s concept of „secondary rules” (rules in which the main norms are recognized). Hund therefore says that for some cultures, for example in parts of Tswana society, secondary rules have only evolved to the point where laws are set in terms of politics and personal preferences. That`s not to say they aren`t „rules.” Hund argues that if we recognize a pattern of development in the constructions of these secondary rules in societies, we can understand how that society constructs its laws and how it differs from societies that relied on an objective and distinct set of rules.  Most customary laws deal with community norms that have long prevailed in society. However, customary law may also address certain areas of international law if certain norms are universally accepted. For example, laws against slavery, piracy, terrorism, etc. These customs are not legally binding, but are still prevalent in society and subject to social sanctions.
These customs do not refer to the circle of social conventions, external decency or style; Rather, they are concerned with the affairs of society itself, with the work that must be done in the application to verify and ensure the conditions necessary for community life. It must be clear and unambiguous what the custom is and how it is practiced. A custom can continue before the courts only if it is not indefinite or uncertain. It must be absolute and objective in theory and practice. Customary law may be subordinated to laws and regulations. It is a recognized source of law that falls within the tradition of civil law. In many countries, one or more types of customary law generally exist alongside formal law. This is what we call legal pluralism. Knowledge of habits and customs is then studied by a minority, especially the religious. This is possible by weakening the power of the rulers over the people. Priests study customs, recognize patterns, understand their relevance, and formalize customs. Most customary laws deal with community standards that have long been established in a particular place.
However, the term can also be applied to areas of international law in which certain norms have been almost universally accepted as correct bases for action – for example, laws against piracy or slavery (see hostis humani generis).