History of the Union Flag

The Union Flag, or Union Jack is the national flag of the United Kingdom. 
It is so called because it combines the emblems of the three countries united under one Sovereign - the kingdoms of England and Wales, of Scotland and of Ireland (although since 1921 only Northern Ireland has been part of the United Kingdom. 
The term Union Jack possibly dates from Queen Anne's time (reigned 1702-14), but its origin is uncertain. It may come from the 'jack-et' of the English of Scottish soldiers; or from the name of James 1 who originated the first union in 1603, in either its Latin or French form Jacobus or Jacques; or, as 'jack' once meant small, the name may be derived from a royal proclamation issued by Charles II that the Union Flag should be flown only by ships of the Royal Navy as a jack, a small flag at the bowsprit. 
The flag consists of three heraldic crossed. The cross of St George, patron saint of England since the 1270's, is a red cross on a white ground. After James I succeeded to the throne, it was combined with the cross of St Andrew in 1606. The cross saltire of St Andres, patron saint of Scotland, is a diagonal white cross on a blue ground. The cross saltire of St Patrick, patron saint of Ireland, is a diagonal red cross on a white ground. This was combined with the previous Union Flag of St George and St Andrew, after the Act of Union of Ireland with England (and Wales) and Scotland on 1 January 1801, to create the Union Flag that has been flown ever since. 
The Welsh dragon does not appear on the Union Flag. This is because when the first Union Flag was created in 1606, the Principality of Wales by that time was already united with England and was no longer a separate principality. 
The Union Flag was originally a Royal flag (when the present design was made official in 1801, it was ordered to be flown on all the King's forts and castles, but not elsewhere). 
It is today flown above Buckingham Palace, Windsor Castle and Sandringham when the Queen is not in residence. The Royal Arms of Scotland (Lion Rampant) is flown at the Palace of Holyroodhouse and Balmoral when The Queen is not in residence. On news of a Royal death, the Union Flag (or the Royal Arms of Scotland (Lion Rampant) where appropriate) will be flown at half-mast. The Royal Standard is never flown at half-mast, as the Sovereign never dies (the new Monarch immediately succeeds his or her predecessor). 
The flying of the Union Flag on public buildings is decided by the Department for Culture, Media and Sport at the Queen's command. The Union Flag is flown on government buildings on days marking the birthdays of the Royal family. Although the Union Flag originated as a Royal flag, it is now also flown by many people and organizations elsewhere in the United Kingdom by long established custom. Its use as an emblem has extended beyond the form of a flag and the Union Flag is frequently depicted on other objects. The Union Flag has particular significance to the Armed Forces. Flags of contrasting colours have been used to show the position of commanders in the field and their armies from early times. The heavy defensive armour of medieval times made recognition symbols essential, and flags or colours were necessary as rallying points for the horse and foot soldiers. 
One example of the historic importance of colours as the focus of loyalty to the Crown (and to the regiment) is the annual Trooping the Colour ceremony at Horse Guards Parade on the Sovereign's official birthday, when The Queen as Colonel-in-Chief of each of the five regiments of Foot Guards takes the salute. As an emblem of 'Her Majesty's Service', the Union Flag is the flag of the Army, which unlike the Royal Navy and the Royal Air Force, does not have its won ensign. The Union Flag as the King's / Queen's Colour was therefore carried into battle, along with their own Regimental Colours, by a number of Army regiments (artillery did not need them, as their rallying point was the right-hand gun; rifle regiments do not carry colours as their original role was as scouts, which required stealth and concealment). British Regimental Colours were carried into battle for the last time in January 1881, when the 58th Foot (now part of the Royal Anglia Regiment) took their Colours into action against the Boers in the Transvaal, South Africa.
Today, Infantry battalions have two colours or flags: the Queen's Colour (a Union Flag with the title and number of the battalion at the centre surmounted by St Edward's Crown, with First and Second World War battle honours on both sides), and the Regimental Colour (featuring the regimental badge and battle honours awarded for campaigns other than the two World Wars).  
Nowadays, the Union Flag is also carried in smaller form on the sides of Army vehicles and worn on uniforms as a distinguishing national emblem. In the Royal Navy, flags and ensigns assumed the same importance as standards and colours in the Army. 
Until 1864, fleets were organised into White, Red and Blue squadrons, but in that year Queen Victoria ordered that the White Ensign - the red cross of St George with the Union Flag in the top left-hand corner - should be carried by all ships of the Royal Navy. The Naval Reserves and the Mercantile Marine were to fly the Blue and Red Ensigns respectively. 
There are also Queen's Colours for the Royal Navy in each of the present-day naval commands, based on the White Ensign, which are carried on suitable ceremonial occasions by naval colour parties. King's or Queen's Colours have also been presented to the other Services. Colours were presented to the Royal Marines (formed as the Admiral's Regiment in 1664) by the Duke of Clarence (the future William IV) in 1827; today there are Queen's and regimental colours for each Royal Marine Commando. 
In 1947, George VI approved the award of King's Colours to the Royal Air Force for carrying on ceremonial occasions: King's or Queen's Colours have subsequently been presented, amongst others, to the Royal Air Force in the United Kingdom in 1951 and the Royal Auxiliary Air Force in 1989.