The Catechism in Questions & Answers

361. What is the meaning of the Ninth and Tenth Commandments?

The core of the Ninth and Tenth Commandments is the statement: “You shall not covet”. This does not prohibit every form of human desire, only the sinful lust after the wife or property of one’s neighbour.
If this desire is directed at that which is dear and valuable to another person, or that which belongs to him, it becomes sinful lust. Then it will have a destructive effect. Desire can develop into greed and most often has its source in envy.