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Since ancient times, sailors try to communicate with flags between ships, but we have to wait until 1738 to see the first digital flag coding created by the French captain Mahé de la Bourdonnais.
In 1790, Lord Richard Howe, from the British Royal Navy, propose The "Howe Code", made with ten coloured flags, six special flags for control characters and a dictionnary of 260 digital entries.
In 1803, the admiral sir Home Popham publish his "Telegraphic Signals" or "Marine Vocabulary". This directory include somes 3000 digital signals corresponding to words, expressions and entire sentences. From 1803 to 1812, Popham extend the signal flags system with a new set of digital flags.

The actual international marine flags signals coding date from 1934, It's used by vessels to send small messages, any flag gets its own alphabetical or digital meaning , but it's rather used as specific meaning, one by one or in group.

1. Alphabetical Signals

Alpha I have a diver down, keep well clear at slow speed.
Bravo I am taking in, or discharging, or carrying dangerous goods.

Charlie Yes (affirmative).
Delta Keep clear of me; I am maneuvering with difficulty.

Echo I am altering my course to starboard.
Foxtrot I am disabled; communicate with me.

Golf I require a pilot.
Hotel I have a pilot on board.

India I am altering my course to port.
Juliet I am on fire and have dangerous cargo on board, stay clear.

Kilo I wish to communicate with you.
Lima You should stop your vessel immediately.

Mike My vessel is stopped and making no way through the water.
November No (negative).

Oscar Man overboard.
Papa In harbor : all persons should report on board the vessel is about to proceed to sea.
At sea : May be used by fishermen to mean: "My nets have come fast on an obstruction".

Quebec My vessel is healthy and I request free practique (clearance).
Romeo received.

Sierra I am operating astern propulsion.
Tango Keep clear of me; I am engaged in pair trawling.

Uniform You are running into danger.
Victor I require assistance.

Whiskey I require medical assistance.
Xray Stop carrying out your intentions and watch for my signals.

Yankee I am dragging my anchor.
Zulu I require a tug.

2. Digital Signals

0
1
2
3
4

5
6
7
8
9

3. Substitution Signals

Substitute 1
Substitute 2
Substitute 3

4. Answer Signal

5. Two Flags Signals

Alpha + Charlie : I am abandoning my vessel.
Bravo + Romeo : I require a helicopter.
Delta + Victor : I am drifting.
Echo + Foxtrot : SOS/MAYDAY has been canceled.
Foxtrot + Alpha : Will you give me my position ?
Juliet + Lima : You are running the risk of going aground.
November + Charlie : I am in distress.
Papa + Delta : Your navigation lights are not visible.
Quebec + Delta : I am going ahead.
Quebec + Lima : Quarantine ship.
Quebec + Tango : You should not anchor. You are going to foul my anchor.
Quebec + Uniform : Anchoring is prohibited.
Quebec + Xray : I request permission to anchor.
Quebec + Substitut 1 : Infectious disease on board.
Romeo + Uniform : Keep clear of me; I am maneuvering with difficulty.
Sierra + Oscar : You should stop your vessel instantly.
Uniform + Mike : The harbor is closed to traffic.
Uniform + Papa : Permission to enter harbor is urgently requested. I have an emergency.
Zulu + Lima : Your signal has been received but not understood.