Mayflower Flag - The Union Flag

This Union Flag (consisting of the crosses of St George and St Andrew) was first used in 1606. It signifies the Union of Scotland and Britain (when James VI of Scotland also became James I of England and Wales).

The symbols have these meanings:

The cross of St. George, the patron saint of England since the 1270s, is a red cross on a white bacground ground.

The cross of St. Andrew, the patron saint of Scotland, is a diagonal white cross on a blue background.

This flag is different from the current Union Flag because it does not include the cross of St. Patrick, the patron saint of (Northern) Ireland, which is a diagonal red cross on a white background.

The official Monarchy web site provides this background regarding the meaning of "Union Jack" - a term often used to describe Great Britain's flag:

The term 'Union Jack' possibly dates from Queen Anne's time (r. 1702-14), but its origin is uncertain.

It may come from the 'jack-et' of the English or Scottish soldiers, or from the name of James I who originated the first union in 1603.

Another alternative is that the name may be derived from a proclamation by Charles II that the Union Flag should be flown only by ships of the Royal Navy as a jack, a small flag at the bowsprit; the term 'jack' once meant small.