Hi Arthur. Decorum and etiquette both mean being polite and having good manners. Propriety means conforming to the customs of polite society.

Etiquette has to do with good manners, making others feel comfortable in social situations, and following the rules of proper behavior. Propriety has to do also with rules of behavior, following proper customs and being appropriate. Decorum also has to do with orderliness, good taste and proper attire, customs and orderliness. All of these deal with good manners, proper breeding (being trained well in social situations) following customs of high standards and knowing how to act in formal social situations. All these social customs deal with behavior when hosting dinners, when you are a guest at dinners, when to send thank you cards, how to conduct yourself around people, how to dress, how to treat others, and how to be gracious and kind, etc.