In modern French, monsieur (plural messieurs) is used as a courtesy title of respect, an equivalent of English mister or sir. It can be abbreviated in M. (plural: MM.), Mssr. (plural: Mssrs.), and rarely M r (plural: M rs), but never Mr., which is only for Mister. See also. Dauphin; Fils de France; Madame Royale; Madame; Petit-Fils de France