Mr. (also, chiefly British: Mr) is an English pre-nominal social title used for a man. It is an abbreviation of Mister, though it is almost never spelt out in normal usage. The plural of Mr(.) is Messrs: pronounced messers, an abbreviation for the French messieurs.
In the United Kingdom, most Commonwealth countries and Ireland, a full stop (in the US, a period) does not generally follow the abbreviated form, which is in line with standard practice for abbreviations in those countries. The US usage, with a period, is likewise in line with standard American practice. See the history section of the article on Abbreviation.
Mister is an alteration of Master; the equivalent female titles, Mrs, Miss, and Ms, are variants of Mistress. After the development of the word Mister for adult males, the title Master was retained and used for boys and young men. In some societies, this is now rare or considered affected, although it is still acceptable and common in Britain and is still used in conservative enclaves in Africa, Australia, India and the United States. See more at master.
In direct address, Mr is usually used with the last name only ("May I help you, Mr Ericson?"). In indirect speech, it can be used with either the last name or the full name ("This is Mr James Ericson."; "Would you please help Mr Ericson?") On envelopes, it is usually used with the full name.
The title of Mr, like any other title, is a term of respect, and therefore the use of Mr to introduce or refer to oneself (as in "my name is Mr Smith") is considered somewhat affected or pretentious in conventional protocol, although such usage is becoming increasingly common.
In American English the title "mister" is sometimes used informally by itself in direct address ("Are you all right, mister?"). In formal usage, the title sir is used ("May I help you, sir?)".
In past centuries, Mr. was used with a first name to distinguish among family members who might otherwise be confused in conversation: Mr. Smith would be the eldest present; younger brothers or cousins were then referred to as Mr. James Smith and Mr. Robert Smith and so on. Such usage survives in family-owned business or when domestic servants are referring to adult male family members with the same surname: Mr. Robert and Mr. Richard will be out this evening, but Mr. Edward is dining in. Such usage is rare.
In the Courts of England and Wales, Judges of the High Court are called, for example Mr Justice Crane. Where a forename is necessary to avoid ambiguity it is always used, for example Mr Justice Robert Goff to distinguish from a predecessor Mr Justice Goff. The female equivalent is Mrs Justice Hallett, not Madam Justice Hallett. In court, they are addressed as My Lord or My Lady. When more than one judge is sitting and one needs to be specific, one would refer to My Lord, Mr Justice Crane. High Court Judges are entitled to be styled with the prefix The Honourable while holding office: e.g. the Honourable Mr Justice Robert Goff. In writing, such as in the law reports, the titles "Mr Justice" or "Mrs Justice" are both abbreviated to a "J" placed after the name. For example, Crane J would be substituted for Mr Justice Crane.
In the Catholic Church permanent deacons (that is, deacons who are not preparing for ordination to priesthood) are properly "Mr" (no "reverend") in correspondence, although it has also become customary to address them simply as "Deacon John Jones."
Herr | Señor | Monsieur | Maighstir (labhair ri) | Signore | Herr