Sir Chandos Stanhope Reade, 8th Baronet

''Sir'' is a formal honorific address in English for men, derived from Sire in the High Middle Ages. Both are derived from the old French "Sieur" (Lord), brought to England by the French-speaking Normans, and which now exist in French only as part of "Monsieur", with the equivalent "My Lord" in English. Traditionally, as governed by law and custom, Sir is used for men titled as knights, often as members of orders of chivalry, as well as later applied to baronets and other offices. As the female equivalent for knighthood is damehood, the female equivalent term is typically Dame. The wife of a knight or baronet tends to be addressed as Lady, although a few exceptions and interchanges of these uses exist.

Additionally, since the late modern period, Sir has been used as a respectful way to address a man of superior social status or military rank. Equivalent terms of address for women are Madam (shortened to Ma'am), in addition to social honorifics such as Mrs, Ms or Miss.

Etymology

''Sir'' derives from the honorific title ''sire''; ''sire'' developed alongside the word ''seigneur'', also used to refer to a feudal lord. Both derived from the Vulgar Latin , ''sire'' comes from the nominative case declension and ''seigneur'', the accusative case declension .

The form 'Sir' is first documented in English in 1297, as the title of honour of a knight, and latterly a baronet, being a variant of ''sire'', which was already used in English since at least c.1205 (after 139 years of Norman rule) 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.

Entitlement to formal honorific address by region

Commonwealth of Nations

The prefix is used with the holder's given name or full name, but never with the surname alone. For example, whilst Sir Alexander and Sir Alexander Fleming would be correct, Sir Fleming would not.

Today, in the UK and in certain Commonwealth realms, a number of men are entitled to the prefix of 'Sir', including knights bachelor, knights of the orders of chivalry and baronets; although foreign nationals can be awarded honorary knighthoods. Honorary knights do not bear the prefix "Sir" nor do they receive an accolade; instead they use the associated post-nominal letters.

Church of England clergy who receive knighthoods do also not receive an accolade and therefore do not use the title 'Sir', but instead refer to their knighthood using post-nominal letters. For example, the Reverend John Polkinghorne, KBE would never be referred to as ''Sir John Polkinghorne''. Clergy of other denominations may use different conventions.

Only citizens of Commonwealth realms may receive substantive knighthoods and have the privilege of the accompanying style. In general, only knighthoods in dynastic orders – those orders in the personal gift of the Sovereign and Head of the Commonwealth (the Order of the Garter, the Order of the Thistle and the knighthoods in the Royal Victorian Order) – are recognised across the Commonwealth realms, along with their accompanying styles.

British knighthoods (in the Orders of the Bath, St. Michael and St. George, and in the Order of the British Empire, along with Knights Bachelor) were known as ''imperial honours'' during the existence of the British Empire. Those honours continued to be conferred as substantive, not honorary, awards by most Commonwealth realms into the 1990s. Since then, though former imperial honours are still awarded by certain Commonwealth realms, many of them have discontinued grants of British honours as they have developed their own honours systems, some of which include knighthoods. Today, British honours are only substantive for British nationals so recognised, including dual nationals, and for nationals of those realms which have retained them as part of their honours systems.

Dual national recipients of British knighthoods who hold British citizenship, such as academic and immunologist Sir John Bell, are entitled to the style of ''Sir'' as their knighthoods are substantive. This may not necessarily be the case for dual nationals who are not British citizens and who instead hold citizenship of another Commonwealth realm. In 1974, Lynden Pindling, the Prime Minister of the Bahamas, recommended dual Bahamian-American citizen Sidney Poitier for an honorary knighthood as an honorary Knight Commander of the Order of the British Empire (KBE), an imperial honour, as the Bahamas did not have its own honours system at the time. Although Poitier, as a Bahamian citizen by descent, was eligible for a substantive ('ordinary') award of the KBE, the Bahamian government preferred the knighthood to be honorary as Poitier was permanently residing in the United States. Knighthoods in the gift of the government of a Commonwealth realm only permit the bearer to use his style within that country or as its official representative, provided he is a national of that country; knighthoods granted by other realms may be considered foreign honours. For instance, Anthony Bailey was reprimanded by Buckingham Palace and the British government in 2016 for asserting that an honorary Antiguan knighthood (which was revoked in 2017) allowed him the style of 'Sir' in the UK.

The equivalent for a female who holds a knighthood or baronetcy in her own right is 'Dame', and follows the same usage customs as 'Sir'. Although this form was previously also used for the wives of knights and baronets, it is now customary to refer to them as 'Lady', followed by their surname; they are never addressed using their full names. For example, while Lady Fiennes is correct, Lady Virginia and Lady Virginia Fiennes are not. The widows of knights retain the style of wives of knights, however widows of baronets are either referred to as 'dowager', or use their forename before their courtesy style. For example, the widow of Sir Thomas Herbert Cochrane Troubridge, 4th Baronet, would either be known as ''Dowager Lady Troubridge'' or ''Laura, Lady Troubridge''.

Barbados

Prior to becoming a republic in November 2021, Barbados awarded the title Knight or Dame of St. Andrew within the Order of Barbados. This practice has now been discontinued, though individuals who received a knighthood or damehood when the country was still a Commonwealth realm may continue to use the titles "Sir" and "Dame" within their lifetimes.

Commonwealth realms

* Knight Commander or Knight Grand Cross of the Royal Victorian Order (KCVO/GCVO)In the personal gift of the Sovereign and Head of the Commonwealth.

=United Kingdom

=
* Baronet (Bt.)
* Knight of the Order of the Garter (KG)In the personal gift of the Sovereign and Head of the Commonwealth.
* Knight of the Order of the Thistle (KT)In the personal gift of the Sovereign and Head of the Commonwealth.
* Knight Commander or Knight Grand Cross of the Order of the Bath (KCB/GCB)
* Knight Commander or Knight Grand Cross of the Order of St Michael and St George (KCMG/GCMG)
* Knight Commander or Knight Grand Cross of the Order of the British Empire (KBE/GBE)
* Knight Bachelor

=Antigua and Barbuda

=
* Knight of the Order of the National Hero (KNH)
* Knight Commander, Knight Grand Cross, or Knight Grand Collar of the Order of the Nation (KCN/KGCN/KGN)

=Australia

=
* Knight of the Order of Australia (AK; for male Australian subjects only; discontinued 1986–2014, reintroduced briefly in 2014, again discontinued in 2015)

=Grenada

=
* Knight Commander, Knight Grand Cross, or Knight Grand Collar of the Order of the Nation in the Order of Grenada (KCNG/GCNG/KN)

=New Zealand

=
* Knight Companion or Knight Grand Companion of the New Zealand Order of Merit (KNZM/GNZM)

=Saint Lucia

=
* Knight Commander of the Order of Saint Lucia (KCSL)

India

As part of the consolidation of the crown colony of India, the Order of the Star of India was established in 1861 to reward prominent British and Indian civil servants, military officers and prominent Indians associated with the Indian Empire. The Order of the Indian Empire was established in 1878 as a junior-level order to accompany the Order of the Star of India, and to recognise long service.

From 1861 to 1866, the Order of the Star of India had a single class of Knights (KSI), who were entitled to the style of 'Sir'. In 1866, the order was reclassified into three divisions: Knights Grand Commander (GCSI), Knights Commander (KCSI) and Companions (CSI); holders of the upper two degrees could use the title 'Sir'. From its creation in 1878 until 1887, the Order of the Indian Empire had a single class, Companion (CIE), which did not entitle the recipient to a style of knighthood.

In 1887, two higher divisions, Knight Grand Commander (GCIE) and Knight Commander (KCIE) were created, which entitled holders of those ranks to the style of 'Sir'. The last creations of knights of either order were made on 15 August 1947 upon Indian independence. All British honours and their accompanying styles were officially made obsolete in India when the Dominion of India became a modern republic in the Commonwealth of Nations in 1950, followed by Islamic Republic of Pakistan in 1956.

The Order of the Star of India became dormant in the Commonwealth realms from February 2009, and the Order of the Indian Empire after August 2010, when the last knights of the orders died.

Nigeria

In Nigeria, holders of religious honours like the Knighthood of St. Gregory make use of the word as a pre-nominal honorific in much the same way as it is used for secular purposes in Britain and the Philippines. Wives of such individuals also typically assume the title of Lady.

Non-Commonwealth Countries

Holy See

Knights and Dames of papal orders may elect the "Sir" or "Dame" prefix with post-nominal letters, subject to the laws and conventions of the country they are in. The Pope, the sovereign of the Catholic Church and Vatican City, delegates the awarding orders of knighthood to bishops and Grand Masters. Their precedence is as follows:

* Supreme Order of Christ (Vacant)
* Order of the Golden Spur (Vacant)
* Order of Pope Pius IX