The Union Jack
The flag of the United Kingdom, known as the Union Jack, is made up of three crosses. The upright red cross is the cross of St George, the patron saint of England. The white diagonal cross (with the arms going into the corner) is the cross of St Andrew, the patron saint of Scotland. The red diagonal cross is the cross of St Patrick, the patron saint of Ireland. St David is the patron saint of Wales.
Susan Sheerin, Jonathan Seath, Gillian White: Spotlight on Britain; Oxford University Press, 1985, page 3