officially called the Union flag, because it embodies the emblems of three countries united under one monarch. The Union Flag is commonly known as the Union Jack, although the exact origin of the name is unclear.
One explanation is that it gets its name from the «jack staff» of naval vessels (a small flagpole at the front of Royal Navy vessels) from which the original Union Flag was flown.
The emblems that appear on the Union Flag are the crosses of the three patron Saints: Wales is not represented on the Union Flag because by the time the first version of the flag appeared, Wales was already the part of England.
The Welsh Flag, a red dragon on a field of white and green, dates from the fifteenth century.
The Union Flag underwent a gradual development. The first one was created in 1606, when England and Scotland were united under one King James I, by combining the flags of St George and St Andrew