The modern plural form is Misters, although its usual formal abbreviation Messrs(.) derives from use of the French title messieurs in the 18th century. Messieurs is the plural of monsieur (originally mon sieur, "my lord"), formed by declining both of its constituent parts separately.
Historically, misterlike Sir or my lordwas applied only to those above one's own status in the peerage. This understanding is now obsolete, as it was gradually expanded as a mark of respect to those of equal status and then to all gentlemen. It is now used indiscriminately.
In past centuries, Mr was used with a first name to distinguish among family members who might otherwise be confused in conversation: Mr Doe would be the eldest present; younger brothers or cousins were then referred to as Mr Richard Doe and Mr William Doe and so on. Such usage survived longer in family-owned business or when domestic workers were 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 today is rare in American culture but still quite common in others as a sign of respect when first names are being used, the last name is not known, or where English is not the mother tongue.
Mr is sometimes combined with certain titles (Mr President, Mr Speaker, Mr Justice, Mr Dean). The feminine equivalent is Madam. All of these except Mr Justice are used in direct address and without the name. In certain professional contexts in different regions, Mr has specific meanings; the following are some examples.
In the United Kingdom, the Republic of Ireland and in some Commonwealth countries (such as South Africa, New Zealand and some states of Australia), many surgeons use the title Mr (or Miss, Ms, Mrs, as appropriate), rather than Dr (Doctor). Until the 19th century, earning a medical degree was not required to become a qualified surgeon. Hence, the modern practice of reverting from Dr to Mr after successfully completing qualifying exams in surgery (e.g., Membership of the Royal College of Surgeons) is a historical reference to the origins of surgery in the United Kingdom as non-medically qualified barber surgeons.
In the United States Military, warrant officers and chief warrant officers are addressed as Mister by senior commissioned officers. In the United States Navy and United States Coast Guard it is proper to use Mister to refer to commissioned officers below the rank of lieutenant commander, or to subordinate commissioned officers, though the use of Mister implies familiarity compared to the use of rank title for an unknown officer.
In the British Armed Forces, a warrant officer is addressed as Sir by other ranks and non-commissioned officers; commissioned officers, particularly of junior rank, should address a warrant officer using his surname and the prefix Mister; for example, "Mr Smith", although often their rank or appointment is used, for example "Sergeant Major", "Regimental Sergeant Major", or "RSM".
In the British Armed Forces a subaltern is often referred to by his surname and the prefix Mister by both other ranks and more senior commissioned officers, e.g., "Report to Mister Smithe-Jones at once" rather than "Report to 2nd Lieutenant Smithe-Jones at once".