Whether the United Kingdom's national flag should be called 'the Union Flag' or 'the Union Jack' remains, to this day, a point of debate.
Although generally, most people accept that either name may be used, some people remain certain that 'Union Flag' is the only correct term.
Some believe 'Union Jack' should be used only when the flag is flown from the jack-staff on the bows of a Royal Naval vessel or, at least, when it refers to the use of the flag by the Royal Navy.
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Debate on the matter is almost as old as the flag itself.
There is also a 'correct' way for the flag to be flown (more on that here), with Downing Street stating a Union Flag painted on a Royal Air Force Voyager was "on properly", after questions were raised online.
Origin of the word 'Jack'
A mystery key to deciding which term to use remains the origin of 'Jack' itself.
However, any attempts to historically consider the question are made more difficult by the fact many of the early records relating to the creation of the flag have been lost.
One theory, according to the Flag Institute, is that 'Jack' comes from the name of James I, who first ordered the flag.
Jacques was French for the name James and was the name the monarch preferred when signing his name.
Another theory among vexillologists – people who study flags – is that the word 'Jack' in middle English refers to a short close-fitting jacket or a protective coat that often bore heralds and, therefore, some believe that the name was transferred to the flag.
However, the most probable reason is that the word was commonly used for something that is small.
A 'jack yard' was a small spar used in attaching a gaff topsail, meaning a 'jack flag' was a small flag, smaller than the ensign – and this name was contracted to 'Jack'.
How the flag is named officially
In the absence of any obvious consensus in which term to use, we can look instead at what names are preferred by the Government, the Armed Forces and other official bodies.
It is sometimes claimed that the Union Flag should be described as the Union Jack only when flown on the bows of a Royal Navy ship.
From its earliest days, the Admiralty often referred to the flag, however it was used, as the Union Jack.
In 1902 an Admiralty Circular announced that either 'Flag' or 'Jack' could be used officially.
On 14 July 1908, a statement was made on behalf of the Government in the House of Lords by the Earl of Crewe in reply to a question asking whether the full Union Jack could be flown on land by every citizen in the Empire.
"I think it may fairly be stated... that the Union Jack should be regarded as the National flag, and it undoubtedly may be flown on land by all His Majesty's subjects."
In June 1933, Home Secretary Sir John Gilmour again stated that the Union Jack was the national flag and might be flown by any British subject on land.
Watch: The RAF unveiled a Typhoon in 2021, completed with Union Flag paintwork.