When a merchant systematically delivers their goods on a specific day without any contract stipulating this, and everyone in the profession considers this habit as normal, we touch upon what a custom is in law. This term, often confused with tradition or mere usage, refers to a legal rule born from the repetition of collective behavior. French law recognizes it as having a real place, even though written law largely dominates the legal system.
Corpus and animus: the two pillars that establish legal custom
Before distinguishing the different categories, it is essential to understand what transforms a simple habit into a legal rule. Custom relies on two cumulative elements, without which it does not exist legally.
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The first is the corpus, meaning the repeated and constant practice of a behavior by a group. An isolated act is never sufficient. There must be repetition over a sufficiently long period for the practice to stabilize. This behavior must also be general within the concerned community, not just the act of a few individuals.
The second element is the animus, sometimes referred to as opinio juris. It reflects the shared conviction that this practice is mandatory. Members of the group do not follow the rule out of convenience or politeness, but because they believe it is imposed upon them. It is precisely this psychological criterion that separates custom from mere usage.
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Have you ever noticed that a commercial usage can be set aside by the parties in a contract? This is because usage has only a supplementary or contractual value. Custom, on the other hand, imposes itself independently of the parties’ will when recognized by the judge. This distinction, clearer in practice than in theory, radically changes the legal significance of a habit.
Understanding the forms of custom in law requires mastering these two pillars, as each category is defined by its relationship to written law.
Custom secundum legem, praeter legem, and contra legem: three relationships to written law
The most operational classification of customs is based on their interaction with the law. Each form plays a different role in the legal order.
Secundum legem: when the law refers to custom
Custom secundum legem occurs when the law itself invites reference to established usages or practices. The Civil Code regularly uses phrases like “according to local usage” or “in accordance with usages.” In this case, the custom derives its strength from the law that integrates it. It does not create an autonomous norm; it complements an existing text that delegates part of its content to it.
Concrete example: in the context of agricultural leases, the notice period or certain operating conditions may depend on local usages to which the Code expressly refers.
Praeter legem: filling the silence of the law
When no text addresses a specific issue, custom praeter legem can intervene to fill this gap. It then functions as a source of law independent of any legislative reference.
This scenario is more common in commercial law. Practices among professionals in the same sector, repeated and perceived as mandatory, can acquire normative force recognized by the courts. The judge verifies that both elements (corpus and animus) are present before applying it.
Contra legem: custom against the law
This third form is the most controversial. A custom contra legem directly contradicts a legal provision. In theory, it exists: a behavior contrary to a text, practiced consistently and perceived as mandatory, fulfills the definition of custom.
In practice, custom contra legem remains marginal in French law. The preeminence of written law in our legalistic system significantly limits its recognition by the courts. It may exist as a social fact without producing legal effects before a tribunal.
Custom in international law: a different logic
The reasoning changes in nature in international law. In the absence of a global legislator capable of imposing texts on all states, custom occupies a much broader place than in the domestic legal order.
International custom relies on the same two elements, but adapted to relations between states:
- A general practice followed by a significant number of states, consistently and over a sufficient duration to establish stabilized behavior
- The opinio juris, meaning the conviction of these states that this practice constitutes a legal obligation and not a mere diplomatic courtesy
- A recognition by international jurisdictions, notably the International Court of Justice, which can acknowledge the existence of a custom and apply it as a binding norm
The International Law Commission emphasizes this framework by highlighting that international custom is an autonomous source of law, distinct from treaties. It is not limited to a “diplomatic usage”: it creates legal obligations enforceable against states, even those that have not signed any written agreement on the subject.
Proof of custom before the judge: who must prove what
Unlike written law, which the judge is presumed to know, custom poses a practical proof problem. Since it arises from practice and not from an official text, its demonstration rests on the party invoking it.
Specifically, the party relying on a custom must establish:
- The existence of an ancient, constant, and repeated practice within the concerned group
- The general nature of this practice (not an isolated behavior)
- The collective conviction of its mandatory character
The means of proof vary by domain. In commercial law, opinions (certificates issued by chambers of commerce or professional organizations) are regularly used to establish the existence of a usage. In international law, diplomatic acts, official statements, and the practices of international organizations constitute accepted evidence.
This burden of proof partly explains why custom remains a secondary source in French law. Written law, accessible and published, offers legal certainty that custom, by its nature oral and fluctuating, does not guarantee with the same certainty. However, the judge has the discretion to recognize or dismiss a custom invoked before them, which keeps this source of law alive in the daily practice of the courts.