British Isles

The first Union Jack was used in April 1604. It was a combination of the national flags of Scotland (St Andrew's blue flag with a white diagonal cross) and England (white with a red cross), and marked the accession of King James VI of Scotland as James I of England and Scotland.

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St. Andrew's flag + St. George's flag = First Union flag

It briefly had an Irish harp on it, but in 1800 it became the Union Jack as waved today, when the red diagonal cross from the Irish flag was incorporated.

King James VI

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First union flag + St. Patrick's flag = The Union Jack

Actually, the "jack" is a small national flag flown from the bow of a ship, so the Union Jack is the national flag of the United Kingdom. The name "Union Jack" is used in all circumstances even though it should only be used in naval terms because of its reference to the "jack" flag.

When the "Union Jack" is raised on the flagpole of Buckingham Palace, it means that the Queen is in London.