Pratique is the license given to a ship to enter port on assurance from the captain to convince the authorities that she is free from contagious disease. The clearance granted is commonly referred to as Free Pratique. A ship can signal a request for "Pratique" by flying a solid yellow square-shaped flag. This yellow flag is the Q flag in the set of International maritime signal flags.
Since flying the Q flag involves a request for boarding by Port State Control, it has also become an invitation to Customs to inspect a vessel for dutiable goods or contraband, as in the "Rich Harvest" case.