The flag of the United Kingdom of Great Britain and Northern Ireland is commonly called the Union Jack. During the reign of Queen Elizabeth I, the English used the cross of St. George, a red cross on a white background, as the flag of England. During that time, the Scottish flag was the Saltire, a white Saint Andrew’s cross on a sky-blue background.
At the death of Queen Elizabeth, James VI of Scotland/united the crowns of the two countries as James I of England. A new flag was necessary and the Union Flag created, based on the flag of St. George, the patron saint of England, and the Saltire cross of St. Andrew, the patron saint of Scotland. As Wales was already united with England at that time, the Welsh dragon was not included. In 1801, when Ireland joined the United Kingdom, the Union Flag was updated to incorporate the cross of St. Patrick, the patron saint of Ireland.
The flag of the United Kingdom, the Union Jack, is, then, a superposition of the flags of St. George, St. Andrew and St. Patrick. This superposition is quite intricate, and often drawn incorrectly. The diagram below illustrates correction construction of the flag which should be twice as wide as it is high.

Which of “Union Jack” or “Union Flag” is the correct name?
The Flag Institute answers as follows: “It is often stated that the Union Flag should only be described as the Union Jack when flown in the bow of a warship, but this is a relatively recent idea. From early in its life the Admiralty itself frequently referred to the flag as the Union Jack, whatever its use, and in 1902 an Admiralty Circular announced that Their Lordships had decided that either name could be used officially.” Such use was given Parliamentary approval in 1908 when it was stated that “the Union Jack should be regarded as the National flag.”
The Royal Navy, however, disagrees: “The national flag of the United Kingdom is worn as a Jack at the bow by all HM ships in commission when alongside or when ‘dressed overall.’ This is the only occasion when it is correctly called the Union Jack, although it is general known by this name through common usage. It is also flown during Courts Martial and is the Distinguishing Flag of an Admiral of the Fleet.”
Which of the Flag Institute and the Royal Navy is right? The Naval Historic Branch at Great Scotland Yard, answered as follows: “Both and neither. A jack is a sea flag, a small flag, generally rather square in its proportions, flown from a flagstaff rigged on the bowsprit or stem of the vessel. The earliest known reference to a ‘jack’ of such a type occurs in 1633, the first reference to the Union (rather than the ‘Britain’ or ‘British’ flag) dating from 1625—the Union Flag and the naval jack are much the same… Technically, all Union Jacks are Union Flags, but not vice versa. It is a fine point and one that is much argued over, but it is beyond question that the habit of treating the two terms as interchangeable developed early and it would not be difficult to multiply instances of individuals who undoubtedly did understand the distinction nevertheless following common usage and using the term Union Jack when Union Flag is clearly meant.
The Royal Navy website is quite right in that the Union Jack flown in the bows of commissioned ships is the only one which really is a jack, but not quite so in that being the only occasion when it is ‘correctly’ so called—because the Flag Institute is right that the use of Union Jack to mean any Union Flag has been sanctioned both by the Admiralty and by Parliament.”