Sir is an honorific used as a title (see Knight) and in several other modern contexts.

It was once used (without the person's name) as a courtesy title among equals, but in common usage it is now usually reserved for one of superior rank or status, such as an educator or commanding officer, or in age (especially by a minor); as a form of address from a merchant to a customer; in formal correspondence (Dear Sir, Right Reverend Sir); or to a stranger (Sir, you've dropped your hat).

The equivalent for a woman when used as a term of address is "madam" or "ma'am".

Origin

Sir derives from the Middle French honorific title sire (messire gave 'mylord'), from the Old French sieur (itself a contraction of Seigneur meaning 'lord'), from the Latin adjective senior (elder), which yielded titles of respect in many European languages. The form sir is first documented in English in 1297, as title of honor of a knight or baronet, being a variant of sire, which was already used in English since at least c.1205 as a title placed before a name and denoting knighthood, and to address the (male) Sovereign since c.1225, with additional general senses of "father, male parent" is from c.1250 and "important elderly man" from 1362.

Formal styling

In formal protocol Sir is the correct styling for a knight or a baronet (the UK nobiliary rank just below all Peers of the realm), used with (one of) the knight's given name(s) or full name, but not with the surname alone ("Sir James Paul McCartney", "Sir Paul McCartney", or "Sir Paul", but never "Sir McCartney"). The equivalent for a woman is Dame (for one who holds the title in her own right). The wife of a knight or baronet is however styled "Lady 1" (e.g. "Lady McCartney", but never "Lady Heather McCartney", which is reserved for the daughter of a duke, marquess or earl).

In the case of a military officer who is also a knight, the appropriate form of address puts the professional military rank first, then the correct manner of address for the individual, then his or her name, e.g.,

• Admiral Sir Bruce Fraser, KBE (after 1941).
• General Sir Thomas Blamey, KBE (after 1941).

This is also the case with academic titles such as professor:

• Professor Sir Patrick Bateson, FRS

With regard to British knighthood, a person who is not a citizen of a Commonwealth realm who receives an honorary knighthood is entitled to use any postnominal letters associated with the knighthood, but not the title "Sir".

Dual nationals holding a Commonwealth citizenship that recognise the British monarch as head of state are entitled to use the styling. Common usage varies from country to country: for instance, dual Bahamian-American citizen Sidney Poitier, knighted in 1974, is often styled "Sir Sidney Poitier", particularly in connection with his official ambassadorial duties, although he himself rarely employs the title.