How the International Code of Signals Flag System Works

The International Code of Signals was adopted by the Fourth Assembly of IMCO, now IMO, in 1965 and has been in force since 1 April 1969. The United States edition is published as Pub. 102, last revised in 2020. The Code covers flag signaling alongside flashing light, sound signals, radiotelephony, and Morse by hand flags, but flags remain the method most mariners recognize first.

Each of the 26 alphabet flags has its own single-letter meaning for urgent or common messages. The flags can also be hoisted in two-letter and three-letter groups, which the Code assigns to more detailed messages covering distress, maneuvering, cargo, medical, and other situations. Numeral pennants carry figures for courses, bearings, positions, and times, while the substitutes and answering pennant manage the mechanics of a signal exchange.

What Single-Letter Signal Flags Mean

Several single-letter signals turn up regularly in port, at anchor, and on passage. The meanings below follow the wording in Pub. 102.

• A (Alpha): I have a diver down; keep well clear at slow speed.
• B (Bravo): I am taking in, discharging, or carrying dangerous goods.
• C (Charlie): Yes, affirmative.
• H (Hotel): I have a pilot on board.
• L (Lima): You should stop your vessel instantly.
• N (November): No, negative.
• O (Oscar): Man overboard.
• P (Papa): In harbor, all persons should report on board as the vessel is about to proceed to sea.
• Q (Quebec): My vessel is healthy, and I request free pratique.
• V (Victor): I require assistance.
• W (Whiskey): I require medical assistance.
• Y (Yankee): I am dragging my anchor.

The Q (Quebec) flag deserves particular attention from cruising vessels. A yacht entering a new country's territorial waters flies Q to request clearance, then replaces it with the host nation's courtesy flag once customs and immigration have cleared the vessel. Every vessel making international passages should carry a Q flag in good condition.

How Substitutes and the Answering Pennant Work

Substitutes solve a practical problem. A vessel with a single set of flags cannot hoist the same letter twice, so the Code allows a substitute to repeat an earlier flag in the same group. The first substitute repeats the top flag of the hoist, the second substitute repeats the second flag, and the third substitute repeats the third. For example, the signal VV is made by hoisting V with the first substitute beneath it. Each substitute can be used only once in a group.

The answering pennant acknowledges signals. A receiving vessel hoists it at the dip when it sees a signal, closes it up once the signal is understood, and lowers it back to the dip when the sending vessel hauls its hoist down. The sending vessel flies the answering pennant on its own after the last hoist to show the signal is complete. Without substitutes and an answering pennant, a set cannot carry out a full Code exchange.

Signal Flag Requirements for Diving Operations

Code flag A carries a regulatory role beyond the Code itself. Under Rule 27(e) of the International Regulations for Preventing Collisions at Sea, a vessel engaged in diving operations that cannot show the full restricted-in-ability-to-maneuver day shapes because of its size must display a rigid replica of Code flag A not less than 1 meter in height, visible all round. Recreational divers in U.S. waters often fly the red diver-down flag with a white diagonal stripe as well, under state boating laws, but that flag does not replace Alpha where the Collision Regulations apply.

Choosing Signal Flag Size and Construction

Flag size should match the vessel and the distance at which the signal needs to be read. Small yachts typically fly flags around 12 by 15 inches; larger yachts and workboats move to 18 by 24 inches or 2 by 2 feet, and commercial vessels and ships dressing overall use 3 by 3 feet and up. ANS code sets run from 12 by 15 inches up to 4 by 6 feet, with pennants and substitutes sized in proportion to the alphabet flags.

Construction matters as much as size, because signal flags spend long periods in sun, wind, and salt spray. The ANS code sets use colorfast, fast-drying heavyweight nylon with four rows of lock-stitching to resist fraying, and a duck heading attached to rope lines. Toggle versions use ash toggles for quick attachment to a signal halyard, while grommet versions suit snap hooks. Full sets ship in a nylon storage bag that keeps the flags in order between uses. Individual pennant flags and alphabet flags are available separately to replace single flags in an existing set.

Frequently Asked Questions

How many flags are in a complete marine signal flag set?

A complete International Code of Signals set contains 40 flags: 26 alphabet flags, 10 numeral pennants, three substitutes, and the answering pennant. The ANS code set includes all 40, supplied with a storage bag.

Are marine signal flags required on board?

The International Code of Signals itself is a communication standard rather than a carriage rule for flags, so requirements depend on vessel type, trade, and flag state. Specific flags are effectively required in certain situations, such as a rigid Code flag A for small vessels in diving operations under COLREGs Rule 27(e) and a Q flag for vessels clearing into a foreign port.

What is the difference between a signal flag and a courtesy flag?

Signal flags carry coded messages under the International Code of Signals. A courtesy flag is the national flag of the country whose waters a vessel is visiting, flown from the starboard spreader as a mark of respect. ANS stocks both, including international country flags for courtesy use.

Which edition of the International Code of Signals should vessels carry?

U.S. vessels typically carry Pub. 102, the United States edition, which includes amendments through the IMO Maritime Safety Committee's 73rd session. The International Code of Signals collection covers the current edition, which lists the full set of single-letter, two-letter, and three-letter signals.

Can signal flags be used to dress a ship?

Yes. Dressing ship strings the full set of signal flags from bow to stern over the mastheads on national holidays, at regattas, and for ceremonial occasions. The flags are arranged for visual effect rather than to spell a message, and the practice is customarily limited to vessels in harbor.

For help choosing flag sizes, building a full code set, or ordering for a fleet, contact ANS at +1 (954) 522-3321 or sales@amnautical.com.