The flag’s design dates back to AD 1606. King James VI of Scotland had inherited the English and Irish thrones in AD 1603 as James I. On April 12, 1606, a royal decree was made regarding a new flag to represent the union between England and Scotland. This was the countercharging of England’s flag (a red cross on a white background, known as St George’s Cross), and the flag of Scotland (a white saltire on a blue background, known as the Saltire or St Andrew’s Cross). This was meant to be used exclusively for maritime purposes. King James also began to refer to a “Kingdom of Great Britaine,” although the union remained a personal one and hadn’t been proclaimed officially.
The current Union Flag dates from a royal proclamation following Britain’s military takeover of Ireland in AD 1801. The contemporary flag combines three crosses―St. George’s red cross, representing the Kingdom of England, the white saltire of St. Andrew for Scotland and the red saltire of St. Patrick representing Ireland.
Then there is the one glaring omission: Wales. Because Wales was annexed by England’s Edward I back in 1282 and then “integrated” during the 16th Century, its place in the flag is represented by the cross of Saint George. Wales isn’t specifically represented in the Union Jack as such, since the English Kingdom had already absorbed Wales into it.