Impressive scale 20th Century Union Flag (Union Jack) formally of the United Kingdom. It is so called because it combines the crosses of the three countries united under one Sovereign.
 
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 Andrew, 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.