In the canon law of the Catholic Church, custom is the repeated and constant performance of certain acts for a defined period of time, which, with the approval of the competent legislator, thereby acquire the force of law. A custom is an unwritten law introduced by the continuous acts of the faithful with the consent of the legitimate legislator. Historically, some ritual and regulatory customs would be recorded in texts known as customaries for use both within particular cathedrals and religious orders or for dissemination among associated ecclesial communities.

Custom may be considered as a fact and as a law. As a fact, it is simply the frequent and free repetition of acts concerning the same thing; as a law, it is the result and consequence of that fact. Hence its name, which is derived from consuesco or consuefacio and denotes the frequency of the action.

In order for custom to become a source of law, it must be approved by the competent legislator. Custom in canon law is not simply created by the people through their constant performance of a certain act, but it is the constant performance of a certain act, with the intention of making a custom, which is approved by the competent legislator, thereby acquiring the force of law. This is because of the Catholic ecclesiological teaching on the constitution of the Catholic Church, which states that Christ constituted the Church by divine delegation of power to the hierarchical authorities; the Church was not created by the consent of the governed, but by the direct will of Christ.

The true efficient cause of an ecclesiastical custom, in as far as it constitutes law, is solely the consent of the competent legislating authority. All church laws imply spiritual jurisdiction, which resides in the hierarchy alone, and, consequently, the faithful have no legislative power, either by divine right or canonical statute. Therefore, the express or tacit consent of the church authority is necessary to give a custom the force of an ecclesiastical law. This consent is denominated legal when, by general statute and antecedently, reasonable customs receive approbation. Ecclesiastical custom differs, therefore, radically from civil custom. For, though both arise from a certain conspiration and accord between the people and the lawgivers, yet in the Church the entire juridical force of the custom is to be obtained from the consent of the hierarchy while in the civil state, the people themselves are one of the real sources of the legal force of custom. Custom, as a fact, must proceed from the community, or at least from the action of the greater number constituting the community. These actions must be free, uniform, frequent, and public, and performed with the intention of imposing an obligation. The usage, of which there is question. must also be of a reasonable nature. Custom either introduces a new law or abrogates an old one. But a law, by its very concept, is an ordination of reason, and so no law can be constituted by an unreasonable custom. Moreover, as an existing statute cannot be revoked except for just cause, it follows that the custom which is to abrogate the old law must be reasonable, for otherwise the requisite justice would be wanting. A custom, considered as a fact, is unreasonable when it is contrary to divine law, positive or natural; or when it is prohibited by proper ecclesiastical authority; or when it is the occasion of sin and opposed to the common good.

A custom must also have a legitimate prescription. Such prescription is obtained by a continuance of the act in question during a certain length of time. No canonical statute has positively defined what this length of time is, and so its determination is left to the wisdom of canonists. Authors generally hold that for the legalizing of a custom in accordance with or beside the law a space of ten years is sufficient; while for a custom contrary to law many demand a lapse of forty years. The reason given for the necessity of so long a space as forty years is that the community will only slowly persuade itself of the opportuneness of abrogating the old and embracing the new law. The opinion, however, which holds that ten years suffices to establish a custom even contrary to the law may be safely followed. In practice the Roman Congregations scarcely tolerate or permit any custom, even an immemorial one, contrary to the sacred canons. In the introduction of a law by prescription, it is assumed that the custom was introduced in good faith, or at least through ignorance of the opposite law. If, however, a custom be introduced through connivance, good faith is not required, for, as a matter of fact, bad faith must, at least in the beginning, be presupposed. As, however, when there is question of connivance, the proper legislator must know of the formation of the custom and yet does not oppose it when he could easily do so, the contrary law is then supposed to be abrogated directly by the tacit revocation of the legislator. A custom which is contrary to good morals or to the natural or divine positive law is always to be rejected as an abuse, and it can never be legalized.

The effects of a custom vary with the nature of the act which has caused its introduction, i. e. according as the act is in accord with (juxta), or beside (præter), or contrary (contra) to, the written law.

Immemorial custom, provided it be shown that circumstances have so changed as to make the custom reasonable, has power to abrogate or change any human law, even though a clause had been originally added to it forbidding any custom to the contrary. To immemorial custom is also attached the unusual force of inducing a presumption of the existence of an Apostolic privilege, provided the said privilege be not reckoned among abuses, and the holder of the presumed privilege be a person legally capable of acquiring the thing in question without first obtaining a special and express Apostolic permission for it. Ferraris notes that no immemorial custom, if it be not confirmed by Apostolic privilege, express or presumptive, can have any force for the abrogation of ecclesiastical liberties or immunities, inasmuch as both canon and civil law declare such custom to be unreasonable by its very nature. In general, it may be said that a valid custom, in both the constitution and the abrogation of laws, produces the same effects as a legislative act.

A special question has been raised by some canonists as to whether the laws of the Council of Trent may be changed or abrogated by custom, even if immemorial, or whether all such contrary customs should not be rejected as abuses. Some of these writers restrict their denial of the value of contrary customs to ordinary, some also to immemorial ones. It is unquestionably a general principle in canon law, that custom can change the disciplinary statutes even of œcumenical councils. The main reason for rejecting this principle in favour of the Tridentine enactments in particular is that any contrary custom would certainly be unreasonable and therefore unjustifiable. It is by no means evident, however, that all such contrary customs must necessarily be unreasonable, as is plain from the fact that some authors allow and others deny the value of immemorial customs in the premises, even when they agree in reprobating the force of ordinary customs.