Quarantine signal

Signals flown by vessels required to show their state of health. By day "Q" of the international code signifies "Ship is healthy-free pratique requested". Flag "Q" over first substitutes signifies that the ship has had cases of infectious diseases or that there has been unusual mortality among rats on board. Flag "Q" over "L" signifies "Ship is infected". By night a vessel entering harbor exhibits a red light over a white light more than 6 feet apart which signifies that the ship is awaiting free pratique.