Before getting into the reason why the UK's flag is called the Union Jack, it's rational to explain its combination first. The United Kingdom consists of four countries: England, Scotland, Wales, and Northern Irelands. Taking this into account, we can now explain how the country's current flag is a combination of three older national flags; St George’s Red Cross of the kingdom of England, St Andrew’s white saltire of Scotland, and St Patrick’s red saltire of the Irelands.
The origin of the flag of Great Britain dates back to 1606 when James VI united England and Scotland. The flag is referred to as Union Jack or Union Flag interchangeably as an indication of the unity of these three older nations.
In other words, if you look at the original flag of Ireland, Scotland’s flag and England’s one too and imagine them one above the other, you’ll see the UK’s current flag vivid and clear, representing their unity.
But that's the Union, what about the Jack? Before the 17th century, the word Jack was used as a description for the maritime bowsprit flag and when warships used to mount the UK flag on the front, it was called "the Jack". In 1801 when King George called the new flag the Union Flag, the Royal Navy, which was powerful at the time, referred to it as the Union Jack, so the name stuck.
Union Jack Upside-Down
Be careful when you hang the flag, as hanging it upside-down is conveyed as an offensive gesture! So always make sure that the red line in the top-left corner is at the bottom of the white cross.
English Flag vs. British Flag
England is a part of Great Britain, while Great Britain is a part of the United Kingdom. England's flag is called the St. George's Cross, while Great Britain's official flag is called Union Flag or the Union Jack as explained earlier.
Now that you know about their flag, you can learn more about their geographical territories from our The Difference Between the UK, Great Britain and England blog article.