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: ['mesə(r)z]), 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?)".

United States usages
"Mr" can be combined with certain titles (Mr President, Mr Speaker, Mr Justice, Mr Dean). The female equivalent is Madam. All of these except Mr Justice are used in direct address and without the name. The title Mr Justice Krever is not used in direct address. In certain professional contexts in different regions, "Mr" has specific meanings; the following are some examples.

Professional titles
In the United States Supreme Court, instead of Mr. or Madam Justice, the current practice is simply to use Justice. However, the Chief Justice of the United States may be referred to as either "Mr. Chief Justice" or "Chief Justice Roberts." In writing, such as law reports, titles are often abbreviated to a "J" placed after the name (e.g. Breyer, J. would be substituted for Justice Breyer). However, the current practice of the Supreme Court is to use "Justice X" in their opinions, but "X, J." in the syllabi preceding. Multiple Justices would be referred to as "X, Y, and Z, JJ." and the Chief Justice is noted as "C.J." The style "The Honorable" can be used in writing or indirect address, though is used without "Justice" (e.g. The Honorable John Paul Stevens). However, "Your Honor" is appropriate in direct address.
In the Courts of England and Wales, Judges of the High Court are called, for example Mr Justice Crane (unless they are entitled to be addressed as Lord Justice). 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.

Judges
In the United Kingdom, Republic of Ireland and Australia, male medical practitioners who have been admitted to any of the Royal Surgical Colleges discontinue use of the "Doctor" title and revert to using Mister. This system (which applies only to surgeons, not physicians) has its origins in the 18th century, when surgeons were barber-surgeons and did not have a degree (or indeed any formal qualification), unlike physicians, who were doctors with a doctoral university medical degree. Female doctors similarly revert to using the title Miss, Mrs or Ms, depending on marital status and preference.

Surgeons
Among Roman Catholic clergy, "Mr" is the correct title and form of address for seminarians and other students for the priesthood. A diocesan seminarian is correctly addressed as "Mr", and once ordained a deacon, is addressed in formal correspondence (though almost never verbally) as Reverend Mister or Rev. "Mr". In clerical religious orders (orders which include or are primarily made up of priests), Mr is the title given to scholastics. For instance, in the Jesuits, or Society of Jesus, a man preparing for priesthood who has completed novitiate but who is not yet ordained is properly, "Mr John Smith, S.J." and is addressed verbally as "Mister Smith" -- this is to distinguish him from Jesuit brothers (who take vows, but who never become priests), and priests (who have received the sacrament of Holy Orders). Orders founded before the 16th Century do not, as a rule, follow this practice: a Franciscan or Dominican, for instance, becomes a friar after novitiate and so is properly Brother, and a Benedictine, for instance, becomes a monk, and is again properly brother, until they reach ordination.
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."
Eastern Catholic clerics have different protocols, usually dictated by ethnic preferences for particular titles.

Roman Catholic clerics
Since the term Mr does not indicate whether a man is married or not, many feminists believed that a woman's title should not indicate marital status either. Ambrose Bierce once satirically proposed that, as a parallel to Miss, the title of unmarried men should be Mush. See a further discussion at Mrs.

Marital status
Other equivalents of Mr include:

For more details on this topic, see Japanese titles. Other uses

Doctor
Mrs.
Ms.
Miss
Mr. (artist) - a contemporary Japanese artist who goes by the name "Mr."
Nurse
Officer
Father
Rabbi
Shri
Captain
Commander
Sheriff
Private