Mr or Mr. is an English pre-nominal social title used for a man too old to be addressed as Master, under the rank of knighthood, and, supposedly though not really in practice, above some undefined level of social status (see below). In Britain, though not in the United States, the title also excludes those who have the title Dr. It is an abbreviation of Mister, though it is almost never spelt out in normal usage. The plural of Mr is Messrs (IPA pronunciation: ), an abbreviation for the French messieurs.
In the United Kingdom, most Commonwealth countries and Ireland, a full stop (in the US and Canada, 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.
When addressing someone directly, Mr is usually used with the last name only ("May I help you, Mr Ericson?"). In other circumstances, 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, or with initials and surname.
The title of Mr, like any other title, is a term of respect, and failure to use it where it might be expected may be interpreted as a sign of (perhaps deliberate) disrespect, though it is commonly not used among close friends or in other familiar settings.
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?)".