Ships have used flags for signalling for centuries. One of the more obscure flags is the Quarantine Flag, the purely yellow flag shown above. It’s a specialised flag like the better-known diver’s flag. Its use has changed over the years; a brief summary of that use comes from here:
There is no signal in the International Code of Signals for ‘quarantine’ (indeed the word ‘quarantine’ does not appear in the code). Today ships signal either ‘ My vessel is “healthy” and I request free pratique’ with a single Q (Quebec) flag or ‘I require health clearance’ with the double signal QQ (Quebec Quebec). Either is correct for a vessel yet to be cleared for pratique (pratique is permission to do business at a port, granted to a ship that has met quarantine or other health regulations). The Q (Quebec) flag is square in shape and pure yellow. Continuing to fly either of these signals indicates a vessel is yet to receive clearance (and is thus effectively in quarantine). Once the local authorities have determined that the ship’s health problems have been resolved and removed a quarantine order, the ship may strike the signal and raise the national ensign for the port they are entering.
In the days of COVID-19, this flag should be getting a workout. So if your vessel (or house, etc.) is under quarantine (like some of the people taking this course) you should strike whatever flag you might be flying and hoist this one.