Communis error facit jus

[Latin, common error makes law.] Another expression for this idea is "common opinion," or communis opinio. In ancient Rome, the phrase expressed the notion that a generally accepted opinion or belief about a legal issue makes that opinion or belief the law. Judges have pointed out that universal opinion may also be universal error. Until the error is discovered, however, the belief continues to be the law. The concept of communis opinio is not especially favored by contemporary U.S. courts.

---

Communis error facit jus. A common error makes law. What was at first illegal, being repeated many times, is presumed to have acquired the force of usage, and then it would be wrong to depart from it. The converse of this maxim is communis error no facit just. A common error does not make law.