The Union Jack
The Union Flag, popularly known as the Union Jack, is the national flag of the United Kingdom.
It is called the Union Flag because it represents the union of the countries of the United Kingdom. It is made up of the individual Flags of three countries all united under one monarch - the countries of England, of Scotland and of Northern Ireland. The Welsh flag, a red dragon on two equal horizontal stripes, white above and green below, does not appear on the flag because when the Union Jack was created, in 1606, Wales was no longer a kingdom but a principality and already united with England.
Saint George's Cross, a red cross on a white ground, represents England; Saint Andrew's Cross, a diagonal white cross (called saltire) on a blue field, represents Scotland; and Saint Patrick's Cross, a diagonal red cross on a white background, represents Ireland.
As the rules of heraldry demand that two colours must never touch each other, when the red cross of England was put onto the flag of Scotland, a white border was added around the red cross.
The position given to St. Andrew's Cross in one quarter is the same as that given to Saint Patrick's Cross in the diagonally opposite quarter; in heraldry this is known as counterchanging. That is why the Union Jack is not symmetrical.