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Meaning of quarantine flag

quar'antine flag"

Pronunciation: [key]
— Naut. Naut.
  1. a yellow flag, designating the letter Q in the International Code of Signals: flown by itself to signify that a ship has no disease on board and requests a pratique, or flown with another flag to signify that there is disease on board ship. Also called
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.