When did the "yellow jack" flag change meanings?

May 22, 2020 4:02 PM

Historically, a vessel coming into port would fly a yellow flag -- called a yellow jack -- to indicate the presence of disease on a ship, to warn port authorities not to board. Yesterday, I learned that the meaning of the flag has reversed, such that in modern times a ship flies a yellow flag to indicate that there's no infectious disease on board. When did it change and why?

Yesterday, a friend pointed me to a dumb thing where Qanon believers are excited that Trump wore a yellow tie, supposedly a sign that there's no danger from the pandemic. "Wait, but doesn't a yellow flag mean there *is* danger from disease?" I looked it up and found out that it used to mean that, but the sense has reversed. The quarantine flag is now the one with a 2x2 grid of alternating black and yellow squares.

Does anyone know when it changed and why? If you have a citation I will update the wikipedia article. Thank you!

posted by chrchr to Society & Culture (3 answers total)

Here is one answer :

"An English Decree dated 1799 prescribes the size of the quarantine flag as "six breadths of bunting", which means six times the size of an ordinary flag. The "London Gazette" from 6 April 1805 published a Decree prescribing in detail the quarantine moorage, limited by yellow buoys topped with a yellow flag. The Decree from 10 October 1806, however, prescribed an "eight breadths of bunting yellow and black flag".

"In 1832, the Nautical Magazine published a Decree, similarly prescribing a flag with yellow and black squares, eight breadths of bunting in size, to be hoisted by the quarantined ships on the coasts of Britain. The signal had to be hoisted on top of the mainmast. The naive lacking the bill of health has to fly on day a big yellow flag with a black ring or a plain black disc in the middle. This flag was tow breadths of bunting in size."

When it was internationally adopted is a different question, I think. You might have luck tracking down editions of the International Code of Signals and figuring out when the Yellow Jack was formally adopted. I am having trouble finding the 1934 edition (though this set of tobacco cards from 1934 indicates that the solid yellow flag = "My vessel is healthy"). Here's 1969's edition (.pdf), which includes the LIMA flag.
posted by MonkeyToes at 6:20 PM on May 22, 2020 [7 favorites]

That's a really interesting question... or maybe two questions: when did the yellow jack start meaning "I am in quarantine and waiting for pratique" rather than "I have disease onboard" (if those two meanings can be clearly distinguished), and when did black/yellow check start meaning "I have disease onboard"?

The 1861 Code of Flotilla and Boat Squadron Signals for the US Navy says "A yellow flag will be hoisted while in quarantine and until pratique is granted". This 1892 book has the same description.

The yellow jack became Q with the introduction of the International Code of Signals. I can't find a scan of the original 1857 version, but this 1882 printing of the International Code of Signals doesn't say much about single-flag signals, just that Q is "quarantine", and has L as blue/yellow check. This 1906 printing gives the modern meanings for Q and L, and the modern black/yellow for L.

1916 Brown's Signalling says (of Q) In foreign waters flown separately, it is hoisted on quarantine hospitals. Hoisted when entering ports it denotes I have a clean bill of health, but am liable to quarantine. And I was reminded of the film The Navy Lark (1959), and the scene with the quarantine flag 32m in...

(Aha, I see MonkeyToes has found a bit more while I was typing this up...)
posted by offog at 6:29 PM on May 22, 2020 [1 favorite]

Also, it doesn't directly bear on the question, but the 1799 Signal-book for the Ships of War is beautiful.
posted by offog at 6:32 PM on May 22, 2020 [8 favorites]