The Catechism in Questions & Answers

355. What is the significance of the Eighth Commandment in the Old Testament?

Originally the Eighth Commandment applied to false statements made in court. Both false accusation and untrue testimony were considered “false witness” in the sense of this commandment. If the court discovered that a witness had made a false statement, this witness would receive the punishment which would have applied to the defendant had he been found guilty (cf. Deuteronomy 19: 18-19).