The Union Jack, symbol of everything British, all began four centuries ago, when James VI of Scotland became James I of England. There was confusion over what flag the navy should fly. On land, the red cross of St George and the white cross of St Andrew were still, respectively, the flags of England and Scotland. But at sea there was a need for a single flag.
On April 12,1606, a proclamation was issued to the navy that stated that all ships "shall bear in their maintop the red cross commonly called St George's Cross and the white cross commonly called St Andrew's Cross, joined together according to a form made by our heralds".
A white border around the red cross was added to this Union flag because the rules of heraldry demanded that the two colours must not be placed on top of each other. It was called "The British Flag" and it was not universally popular. The Scots were upset that the red cross was laid over the white cross and the Welsh weren't allowed to contribute because their principality had already been annexed by the English.
By the end of the 17th century, it was known colloquially as the Union Jack.
The origin of the word "Jack" in the title is uncertain, but probably stems from another royal proclamation, this one by Charles II, that the flag should be flown only by ships of the royal navy as a jack, a small flag at the bowsprit.
Note that the proportions of the flag have gradually changed over the years to its current proportions of one to two. This was caused by a steady decrease in the width of the cloth used to make the flags. The specifications were in the form of so many widths high by so many yards long. This meant that as the width of cloth reduced the flag apparently became longer and longer. The blue coloration has also deepened from it’s original sky blue.
Queen Victoria used the Royal Standard rather than the Union flag as her official flag. The result of that decision was that the standard became the personal banner of the sovereign, while the red, white and blue colours emerged, by the time of her death, as "the people's flag". By 1926, the flag had been commercialised.
After VE-Day and the coronation of Elizabeth II, it became ubiquitous for every national celebration, while the lowering of it in India, Aden and Hong Kong, among other places, came to symbolise Britain's colonial decline.
Four years after the Act of Union with Ireland in January 1801, when the red diagonal cross of St Patrick was added, Lord Nelson ordered the Union Jack to be flown by the British fleet at Trafalgar as a battle flag. It has been flown on all British ships of war and naval bases ever since.
