CODE OF CANON LAW
BOOK I. GENERAL NORMS
CUSTOM (Cann. 23 - 28)
Can. 23 Only that custom introduced by a community of the faithful and approved by the legislator according to the norm of the following canons has the force of law.
Can. 24 §1. No custom which is contrary to divine law can obtain the force of law.
§2. A custom contrary to or beyond canon law (praeter ius canonicum) cannot obtain the force of law unless it is reasonable; a custom which is expressly reprobated in the law, however, is not reasonable.
Can. 25 No custom obtains the force of law unless it has been observed with the intention of introducing a law by a community capable at least of receiving law.
Can. 26 Unless the competent legislator has specifically approved it, a custom contrary to the canon law now in force or one beyond a canonical law (praeter legem canonicam) obtains the force of law only if it has been legitimately observed for thirty continuous and complete years. Only a centenary or immemorial custom, however, can prevail against a canonical law which contains a clause prohibiting future customs.
Can. 27 Custom is the best interpreter of laws.
Can. 28 Without prejudice to the prescript of can. 5, a contrary custom or law revokes a custom which is contrary to or beyond the law (praeter legem). Unless it makes express mention of them, however, a law does not revoke centenary or immemorial customs, nor does a universal law revoke particular customs.