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Jacks

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Prior to the early-18th Century, the only foresail generally carried was a square spritsail mounted below the bowsprit (and occasionally one mounted above), however, this was gradually replaced by the much more efficient triangular foresail. The bending on of a triangular sail between foremast and bowsprit meant the this last was no longer clear and could not, therefore, any longer bear a jack-staff and flag when under way.

The privilege of wearing a Union Jack at sea was (and remains) the exclusive right of the Royal Navy, and they weren't about to give it up because of a 'minor' inconvenience like a sail getting in the way. Thus the convention of raising it only whilst at anchor developed.
Christopher Southworth, 4 September 2003

I'm in the process of writing something on the history and status of the U.S. jack, which is why this subject interests me at the moment. In my view, the U.S. jack is not the national flag, but it is a national flag. Its use to cover deceased sailors is one of many pieces of evidence suggesting that in the 19th century it was seen that way. U.S. warships used the jack in all the same ways that Royal Navy warships used their Union Jack, including as an additional mark of nationality in battle and as a covering for the capstan when the captain used it as a lectern for conducting church services.
Joe McMillan, 12 September 2003

Perrin (Page 60) places the general introduction the jack on the bowsprit "a year or two before" the Proclamation of 1634. He also says, however, that Flags had no doubt, been carried on the bowsprit from the time when the time when that spar was first invented", and cites the first known instance of this as being 19th July 1545. The first use of the term "jack" to denote a flag is given on P.61 as 3rd July 1633.
Christopher Southworth, 12 September 2003

I see basically four kinds of (national)