Maritime Signal Flags and Meanings (ICS and NATO)

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Global flags expert Tareth Peacock goes over the many international nautical flags & their marine meanings

Co-authored by Dan Hickey

Last Updated: April 6, 2026 Fact Checked

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• Letter Flags
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• Number Flags
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• Other Flags
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• How are they used?
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• What is the ICS?
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• Tips

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All of those colorful flags you see on ships and vessels aren’t just for decoration—each one represents a letter, number, or other distinct meaning, and when combined, they’re used to send complex messages to other boats nearby. In this article, we partnered with a global flags expert to explore what all of the letter and number flags mean according to the International Code of Signals (ICS) and the North Atlantic Treaty Organization (NATO), as well as how these flags are used to communicate.

What do the International Code of Signals flags mean?

Global flags expert Tareth Peacock says the International Code of Signals (ICS) uses flags with letter and number designations to send messages to other ships. Each flag represents a letter, as well as a separate message (for example, the C flag also means “yes”). Flags can be combined to send complex messages.

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Section 1 of 5:

ICS Letter Flags & Meanings

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1. 
   These nautical signal flags are universally understood by vessels for communication. In the International Code of Signals (ICS), Peacock says each flag represents both a letter of the alphabet (A-Z) and a specific meaning or message. (The latter is more common since spelled-out words can be unclear). Peacock notes that the flag meanings are internationally recognised, allowing ships of different nationalities to communicate quickly without relying on spoken language. The letters are read in the NATO phonetic alphabet (Alpha, Bravo, Charlie, etc.) to prevent misunderstandings.

   • A (Alpha): “I have a diver in the water; remain well clear at slow speed.”
     • With numeric flags: Azimuth or bearing (directional headings)
   • B (Bravo): “I am taking on, discharging, or carrying dangerous (explosive) cargo.”
   • C (Charlie): “Yes” or “affirmative.”
     • With numeric flags: Magnetic bearing (angle between magnetic north and an object)
   • D (Delta): “I am experiencing difficulty with maneuvering; keep clear.”
     • With numeric flags: Date
   • E (Echo): “I am directing my course to starboard.”
   • F (Foxtrot): “I am disabled; communicate with me.”
   • G (Golf): “I require a pilot.”
     • With numeric flags: Longitude (in degrees and minutes)
   • H (Hotel): “I have a pilot on board.”
   • I (India): “I am directing my course to port.”
   • J (Juliett): “I am on fire and carrying dangerous cargo; keep clear.”
   • K (Kilo): “I wish to communicate with you.”
     • With numeric flag 1: “By semaphore” (hand flags or arms)
     • With numeric flag 2: “By loud hailer” (megaphone)
     • With numeric flag 3: “By Morse signaling lamp”
     • With numeric flag 4: “By sound signals”
   • L (Lima): “You should stop your vessel immediately.”
     • With numeric flag: Latitude (in degrees and minutes)
   • M (Mike): “My vessel is stopped; making no way.”
   • N (November): “No” or “negative.”
   • O (Oscar): “Man overboard.”
   • P (Papa): “All personnel return to ship; proceeding to sea.” (Used in port.)
   • Q (Quebec): “Ship meets health regulations; request clearance into port.”
   • R (Romeo): No international meaning.
     • With numeric flag: Distance (range) in nautical miles
   • S (Sierra): “Moving astern.”
     • With numeric flag: Speed (velocity) in knots
   • T (Tango): “Engaged in trawling; keep clear.”
     • With numeric flag: Local time (in hours and minutes)
   • U (Uniform): “You are running into danger.”
   • V (Victor): “I require assistance.”
     • With numeric flag: Velocity in kilometers per hour
   • W (Whiskey): “I require medical assistance.”
   • X (X-ray): “Stop carrying out your intentions and watch for my signals.”
   • Y (Yankee): “I am dragging anchor.”
   • Z (Zulu): “I require a tug.”
     • With numeric flag: Coordinated Universal Time (UTC) in hours and minutes

   Meet the wikiHow Expert

   Tareth Peacock is a global flags expert who has over 5 years of experience creating content about flags on his YouTube channel, Flags in Focus.
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Section 2 of 5:

Number Flag Meanings

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1. 
   1
   ICS number flags Peacock explains that ICS also uses number flags from 0 to 9. He says the numerical pennants are often flown with letter flags to convey coordinates, bearings, or specific coded references. For example:

   • A G (Golf) flag followed by numbers indicates longitude. G2367 would be read as “Longitude is 23°67'” (the first two digits indicate degrees, and the second two digits indicate minutes).
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   2
   NATO number flags Ships belonging to NATO member countries use different, square flags to communicate numbers (instead of the standard triangular pennants of the ICS). This is so member vessels can send secure, coded messages when radio communication is not available or secure.
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Section 3 of 5:

Other Maritime Flag Meanings

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1. 
   1
   Answering pennant The answering pennant communicates that another ship’s message has been received and understood. The pennant is hoisted halfway up the signal halyards when the message is received, and all the way to the top once it’s been interpreted and understood.
2. 
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   Storm warning flags Naval stations, marinas, and some vessels will fly storm warning flags to alert sailors of impending bad weather (although electronic or radio warnings are more common these days). The basic storm warning flags include:

   • One red pennant: Small Craft Advisory (winds up to 38 miles per hour that are dangerous to small vessels)
   • Two red pennants: Gale Warning (winds from 39-54 miles per hour)
   • One red square flag with a black center: Storm Warning (winds from 55-73 miles per hour)
   • Two red square flags with a black center: (winds of 74+ miles per hour)
3. 
   3
   Distress flags When the N (November) and C (Charlie) flags are hoisted together (“No/negative” + “Yes/affirmative”), it means the ship is in distress. Generally, these flags should only be hoisted when the ship is in imminent danger and requires immediate assistance (like in cases of sinking, fire, collision, or severe medical emergencies).
4. 
   4
   Substitute pennants Substitute pennants indicate that specific flags in a sequence are meant to be repeated. By using substitute pennants, vessels don’t have to carry multiple sets of flags to communicate the same letter or number twice:

   • First substitute: Substitute for or repeat the first flag (or series of flags) in the hoist
   • Second substitute: Substitute for or repeat the second flag in the hoist.
   • Third substitute: Substitute for or repeat the third flag in the hoist.
   • Fourth substitute (NATO only): Substitute for or repeat the fourth flag in the hoist.
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Section 4 of 5:

How are maritime signal flags used?

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1. 
   Ships can fly 1 to 7 flags in a row to communicate with other vessels. Peacock says the predefined flag meanings allow a vessel to communicate a complete message, a navigational warning, a health declaration, or a request for assistance. If a more complex message is needed, ships can combine flags into multi-flag hoists. Peacock says these are read top to bottom and cross-referenced against the ICS codebook. The codes cover everything from medical emergencies to requests for a pilot. The number of flags used indicates the type or urgency of the message. In general:

   • 1-flag signals: Used to urgent or common signals (like the individual letter flag meanings from above).
   • 2-flag signals: Used for distress or maneuvering signals.
   • 3-flag signals: Used for compass headings, relative bearings, standard times, verbs, punctuation, or general code and decode signals.
   • 4-flag signals: Used for geographical signals and names of ships.
   • 5-flag signals: Used to relate the time and position of the ship.
   • 6-flag signals: Used to indicate main cardinal directions in latitude and longitude signals.
   • 7-flag signals: Used for longitude signals that contain more than 100º.
   • Occasionally, the letter designation of each flag will be used to spell out a word, but this is not very common since miscommunications are more likely.
   • Peacock adds that standardised two-, three-, and four-flag codes represent complete phrases (such as distress, navigation instructions, and medical needs).
   • Peacock notes that a single flag hoisted on a halyard (a line running up the mast) is the most common signal.
   • Substitute (or repeater) flags can also be used, says Peacock. This allows a letter to appear more than once in a hoist without needing duplicate physical flags, which is a practical solution given most vessels’ limited flag inventory.
   • Note that NATO may use its own classified codes in addition to commonly recognized ICS codes.

Section 5 of 5:

What is the ICS?

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1. 
   The International Code of Signals oversees maritime communications. According to Peacock, the ICS (also called INTERCO) is the globally standardised system that governs maritime communication using flags, radio, and other signalling methods. It provides a shared “language” for ships worldwide that helps overcome language barriers and ensure clear communication in critical situations (like emergencies, hazards, port entry procedures, etc.).

   • The ICS was developed in the 19th century and is now maintained by the International Maritime Organization, says Peacock.
   • While radio and satellite communication are used to handle most routine operations, Peacock says flag signals remain a recognised backup system. He notes that they’re often used in port entry procedures, and they carry ceremonial significance in naval and maritime traditions. They’re also important in radio silence situations.
   • Peacock notes that the ICS remains an essential international standard due to its simplicity, visibility, and reliability, even in the modern era of satellite and digital communication.
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