Each of these words, as well as etiquette, refers to the requirements and demands of behavior in so-called polite society. Decorum involves the idea of dignity and reserve in speech, dress, and actions: "This school for foreign officers stresses the importance of decorum at all formal functions." Propriety, a more general term, refers to established conventions and applies to matters of taste and morals as well as dignity. It is usually used in the plural: "When in a foreign country, try to observe the proprieties expected of a well-bred American."
Dictionary of problem words and expressions. Harry Shaw. 1975.