International Code of Signals (INTERCO): Letters, Numbers, Combinations & Canal Signal Rules
The International Code of Signals (INTERCO) is the standardized visual signaling regime adopted under IMO Resolution A.80(IV) and mandated aboard commercial vessels under SOLAS Chapter V (Regulation 21). Modern visual signaling governs vital vessel status reporting, Free Pratique quarantine clearance, and mandatory operational flags required in strategic canals and straits (Suez, Panama, Kiel, and the Turkish Straits).
1. Ship Flag Etiquette, Sovereignty & National Ensigns
Breaching flag etiquette within territorial waters constitutes an infringement on coastal state regulations. Deficiencies can lead to Port State Control (PSC) citations, delays in obtaining Free Pratique, or substantial port administrative fines.
National Ensign (Flag State)
Flown at the stern flagstaff or gaff. Mandatory when entering and departing ports, transiting canals and straits, and in neutral waters when in visual range of naval warships. Standard display hours are from sunrise to sunset (continuous in selected jurisdictions like Mexico).
Courtesy Flag (Host Country)
Flown at the starboard yardarm of the foremast or signal mast as a mark of respect toward the host state. The courtesy flag must never exceed the size of the vessel's stern ensign. Hoisting two national flags on the same halyard is strictly prohibited.
- Saluting Warships: A merchant vessel salutes first by dipping its ensign to one-third the height of the staff. The warship replies in kind, after which the merchant ship hoists its flag close up.
- National Mourning: When a host nation declares national mourning, vessels in port must fly flags at half-mast. The flag is hoisted close up first, then lowered to the half-mast position.
2. Mandatory Single-Letter Operational Flags
3. Numeral Pennants in INTERCO (0–9)
Numerical signals are shaped as trapezoidal pennants. They transmit bearings, depths, times, speeds, geographical coordinates, and assigned berth numbers.
4. Interactive INTERCO Directory & Flag Hoist Builder (A–Z)
Use our interactive bridge tool: type any Latin text to generate a visual signal hoist on the halyard, or click any flag below to inspect its Morse code and operational notes.
5. Two-Letter and Alphanumeric Code Combinations
| Code | Significance | Operational Context |
|---|---|---|
| NC | «I am in distress and require immediate assistance» | Official COLREGs Annex IV visual distress signal. |
| UW / UW1 | «I wish you a pleasant voyage» / Reply: «Thank you, same to you» | Traditional marine courtesy exchange when clearing port. |
| DX | «I am sinking» | Critical loss of buoyancy alert. |
| IT | «I am on fire» | General vessel fire emergency signal. |
| MA | «I require urgent medical advice» | Initiation of TMAS telemedical consultations. |
| K-series (K1–K9) | «I wish to communicate with you»: K6 (flags), K8 (2182 kHz), K9 (VHF Ch. 16) | Specifying the radio or visual communication channel. |
6. Specific Flag Regulations in International Canals & Straits
Canal and strait navigation authorities enforce dedicated visual signal mandates under their local Rules of Navigation. Non-compliance results in loss of transit convoy slot and heavy fines.
Suez Canal (SCA Rules)
- Flag G (Golf): Approaching Port Said or Port Thewfik roadstead to call the canal pilot.
- Flag H (Hotel): Flown continuously while canal pilots are on board.
- Flag B (Bravo): Compulsory for petroleum, LNG/LPG tankers, and bunkering operations.
- Convoy Number Hoist: Visual flag hoist displaying the assigned position in the convoy.
Panama Canal (ACP Regulations)
- Flag H (Hotel): Hoisted continuously; Panama Canal pilots assume control of vessel navigation.
- Pennant 0: Vessels ready for lock entry without transit beam/draft restrictions.
- Flag B (Bravo): Required for dangerous cargo and fueling at Balboa or Cristobal anchorages.
- Panamanian Courtesy Flag: Flown on the foremast from ocean entry to exit.
Kiel Canal (Nord-Ostsee-Kanal)
- Traffic Groups (Verkehrsgruppen): Vessels signal Groups 1 to 6 based on dimensions and draft to coordinate passing at canal sidings (Weichen).
- Flag B: Vessels carrying gas or dangerous goods, and uncleaned ballast tankers.
- Flag H: Pilot and canal helmsman (Kanalsteurer) on duty.
Turkish Straits (Bosphorus & Dardanelles)
- Flag B (Bravo): Mandatory day-signal for vessels over 150m LOA carrying dangerous liquid cargoes.
- Flag H: Displayed when navigating with Turkish VTS authorized pilots.
- Turkish Courtesy Flag: Flown on the starboard yardarm throughout transit of the Sea of Marmara and straits.
7. Substitute Pennants: Duplicating Characters
Standard bunting lockers contain only one set of flags and pennants. Substitutes duplicate characters within a single hoist:
- 1st Substitute: Duplicates the top flag in the hoist.
- 2nd Substitute: Duplicates the second flag from the top.
- 3rd Substitute: Duplicates the third flag from the top.
Example: To hoist 1100: Pennant 1 → 1st Substitute → Pennant 0 → 3rd Substitute.