On 12 April 1606 James I gave the order for a new flag to be created to symbolise the unity between England and Scotland.
Why? When James I ascended the throne three years earlier England and Scotland were two entirely separate kingdoms. He had been King of Scotland, titled James VI. When Elizabeth I, Queen of England died childless, as her cousin the crown of England passed to him and he became James I of England.
Complicated eh! That said, both countries now shared the same monarch.
James wanted to make it super simple and take further steps to unite the two countries under one monarch (him of course!), a common language and a common religion.
A flag seemed a great place to start.
Again, keeping it simple the new flag combined the crosses of St George and of St Andrew. The result was the Union Jack, Jack being a short form of Jacobus, the Latin version of James.
And being practical, this was for maritime use - at sea, super-important to be able to identify ships by their flag.
Earlier, in October 1604 James had decreed that he would in future be known by the style and title of King of Great Britain and not by the divided names of England and Scotland.
However, the flag and title change was as close as he got to achieving his dream - much to his frustration, the perfect ‘union’ into one state would wait over another 100 years to 1707.
The Commonwealth created a new ensign for English warships, which dropped the cross of St Andrew altogether and added a yellow harp for Ireland. In 1658 Oliver Cromwell as Lord Protector put out a Great Union flag, which combined the crosses of St George and St Andrew with the Irish harp.