When a farmer from Gers resolves a right-of-way dispute with his neighbor by referring to a locally transmitted practice that has been passed down orally for several generations, he is applying a custom. Not a text voted on by Parliament, not a decree published in the Official Journal: a rule born from collective repetition and the shared conviction that it is binding.
Custom remains a source of French law, even if written law dominates the legal system. To understand how it works, one must first distinguish its different forms, then assess what separates it from mere usage.
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Contractual algorithms and digital custom: an unprecedented contra legem form
In recent years, contract management platforms have been using predictive algorithms to propose standard clauses, adjust payment deadlines, or trigger automatic penalties. These practices are repeated on a large scale, become predictable, and the parties eventually come to regard them as the norm. We find ourselves with a mechanism that structurally resembles a custom: a repeated behavior (corpus) and a conviction of its obligatory nature (opinio juris).
The fundamental difference lies in the origin. A classic custom emerges from an identifiable human group in a given territory. Here, it is an algorithm that generates the practice, without a human collective having consciously initiated it. If this algorithmic practice contradicts a legal provision (for example, by imposing payment deadlines shorter than the legal minimum), we shift to a new form of contra legem.
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Some jurisdictions are beginning to integrate practices from online platforms as implicit contractual clauses, without requiring the demonstration of a traditional opinio juris. To delve deeper into the different forms of custom and their relationship with the law, it is important to keep in mind that this digital hybridization calls into question the very definition of customary law.
Custom secundum legem, praeter legem, and contra legem: three concrete relationships to the law
In legal terms, the classification of a custom depends on its relationship with written law. This is not a theoretical exercise: the qualification directly determines whether a judge can apply it or must set it aside.
Custom secundum legem: when the law refers to usage
This is the simplest case. The Civil Code itself, in several articles, explicitly refers to usages. In rural law, agricultural leases are often interpreted in light of local practices recognized by law. The judge does not have to question whether the custom is legitimate: the text expressly authorizes him to refer to it.
Custom praeter legem: filling the silence of the law
When no text addresses a situation, custom can step in to fill the gap. In commercial law, many professional usages govern relationships between merchants without any law codifying them. The trade name, for example, benefits from protection largely based on customary practices.
The law of June 18, 2025, on the modernization of contract law explicitly codified certain emerging professional usages in labor law. This text illustrates a movement of hybridization between law and custom: the legislator is gradually absorbing practices that originated outside the written framework.
Custom contra legem: practice against the text
This is the most debated form. A custom contra legem directly contradicts an existing legal provision. In theory, in a legalistic system like ours, it should not prevail. In practice, some usages persist despite the law. The classic example remains the tolerance of certain commercial practices contrary to the Commercial Code, maintained by collective habit.
The French judge generally refuses to prioritize a custom contra legem. Responses vary on this point depending on the jurisdictions and subjects, but the principle of the primacy of written law remains the dominant framework.
Corpus and opinio juris: the two conditions for a usage to become custom
Usage and custom are often confused. The distinction rests on two cumulative elements that must be concretely verified:
- The corpus (material element): a repeated, constant, public, and ancient practice. It is not enough for a few actors to apply it occasionally. The repetition must be observable over a sufficient duration and concern an identifiable group.
- The opinio juris (psychological element): the collective conviction that this practice is obligatory, that it has the force of a rule. It is this criterion that separates custom from mere usage of convenience or politeness.
- A third, less formalized criterion concerns generality: the practice must apply to the entire group or territory concerned, not just to a few individuals.
In rural law, local mediation protocols involving agricultural chambers have helped reduce disputes over customary rights of way. These mediations rely precisely on verifying the corpus and opinio juris to determine whether a local practice has the value of custom or remains a simple informal arrangement.
Custom in French law and Swiss law: a difference in hierarchy that changes everything
In France, custom occupies a subsidiary rank. It only intervenes if the law allows it (secundum legem), if the law is silent (praeter legem), or in rare contested cases (contra legem). The Constitution of 1958 and the Civil Code place written law at the top of the hierarchy of internal norms.
The Swiss system adopts a different approach. In the absence of an applicable law, custom plays a priority supplementary role even before the judge can resort to other methods of interpretation. This difference in hierarchy directly influences how practitioners draft their contracts and anticipate disputes.
This comparison sheds light on the ongoing French reforms, particularly regarding smart contracts. If France were to recognize algorithmic practices as customary sources, the Swiss model would offer a more flexible framework than the current legalistic system.
Custom is not a medieval relic frozen in law textbooks. Between rural rights of way still governed by ancestral practices and contractual clauses generated by algorithms, it continues to produce law, sometimes at the margins of written law.
The real practical question for jurists remains that of proof: demonstrating that a practice meets the conditions of corpus and opinio juris, whether this practice is upheld by a village or by a digital platform.