We’re locking horns on this flag issue. Is the Union Jack solely nautical?
Hope it doesn’t come to blows. Is there alcohol involved?
AFAIK Union Flag is the official term and Union Jack the unofficial. Nautical Flags would be the white, blue and red ensign.
Union Flag is the official name. Union Jack is when it’s on a ship, a Black Jack was a pirate flag ( not the skull and crossbones of movies )
In British maritime law and custom, an ensign is the identifying flag flown to designate a British ship, either military or civilian. Such flags display the United Kingdom Union Flag in the canton (the upper corner next to the staff), with either a red, white or blue field, dependent on whether the vessel is civilian, naval, or in a special category. These are known as the red, white, and blue ensigns respectively.
A Jack is a flag flown from a short jackstaff at the bow of a vessel, while the Ensign is flown on the stern.
Another example on HMS Hurwoth:
- The White Ensign flown on the aft to identify the ship as a British warship
- The Naval Jack flown on the Jackstaff on the Bow.
Differences: Naval Ensigns are usually flown on active ships at any given time. Naval Jacks are flown on occasion. Not every warship has a naval jack. Sometimes the country in question doesn’t have one, sometimes it’s due to the construction of the ship. Aircraft carriers i.e. tend to not having a big metal pole for the naval jack in the middle of the flight deck.
Funfact: if you call the RN Naval Jack the “Union Jack”, you’re in for a detailed lecture about why it isn’t called that at a high level of decibels to make sure you really understand this most important matter.
It is disputed.
it is often stated that the Union Flag should only be described as the Union Jack when flown in the bows 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. In 1908, a government minister stated, in response to a parliamentary question, that “the Union Jack should be regarded as the National flag”
I was arguing nautical only, by the way. However, it looks like I could have been wrong.
I think you can make a case discerning the two by a very small detail: the national flag has a ratio of 1:2 whereas the flag flown on the Jackstaff has a ratio of 3:5.
We do argue about some random stuff. Should really stick to tanks.

I think you can make a case discerning the two by a very small detail: the national flag has a ratio of 1:2 whereas the flag flown on the Jackstaff has a ratio of 3:5.
I vaguely recall a novel where the flag was hung upside down so a guy (Sherlock Holmes?) knew that something was amiss.
EDIT: Or afoot. Pissibly both.
You could say it raised a (red) flag.
Google has spoiled a lot of the endless pointless arguments of the good old days.

Google has spoiled a lot of the endless pointless arguments of the good old days.
The trick is not to look stuff up immediately. Argue it out first.