Introduction to Code Signalling Flags

Ever wondered what those brightly-colored flags on ships mean? Apart from naval jacks, which denote which nation a vessel is sailing under, you may also spot a series of pennants in vibrant red, yellow, blue, and white colors. These flags are a part of the International Code of Signals, a widely-used system for communicating over long distances at sea.

What are Nautical Signal Flags?

Code signal flags are used by vessels to communicate with one another using visual methods, similar to flashing Morse code with a lamp or semaphore flags. Signalling flags can also be used when radio isn’t an option.

Today, signalling flags can be used to communicate with ships at sea, even if the crews of both vessels speak entirely different languages. The modern nautical flag alphabet has 26 letter flags, 10 numeral flags, and 4 substitutes. A specialized vocabulary exists, in which specific combinations of different signalling flags are used to communicate messages.

How are Code Signal Flags Used?

Code signal flags are typically flown from the mast of a ship. To initiate contact with another boat, a captain can choose to fly the Kilo flag, which translates as “I wish to communicate with you”. Further communications are then conducted using the signal flags and their corresponding vocabulary book, which lists the meanings for all possible flag combinations. The Code also includes a special substitute flag flown at the end of each message to show that the message was written using the International Code of Signals.

Signalling flags were sometimes used to spell out whole messages in the past, but this took too long and was prone to mistakes. Today, most maritime messages are limited to a single flag in the marine flag alphabet, which communicates a specific, urgent message. Check out our Code Signal Flag page to see what message each flag represents. 

A series of two or three code flags can be used for more complex messages, in addition to substitute flags. Substitute flags, also called ‘repeaters’, are used to repeat a flag in the specific position. For example, the Second Repeat flag at the end of a message would mean the signal flag in the second position should be repeated.

History and Development of Code Signals

The use of signalling flags at sea dates back centuries, as far back as 1530! However, no signal flag alphabet for complex messages existed at this time—instead, a flag would be raised when the fleet admiral wanted the captains of other ships to come aboard for further instructions. Over the following century, the British Royal Navy constructed a rudimentary method of conveying messages by flying flags on certain parts of a ship.

James II of England came up with the idea of using flags with specific patterns to communicate messages at sea in the mid-17th century. Admiral Lord Howe created a system for communicating numbers in 1790, along with ‘substitutes’ that indicated how many times each numeral should be repeated. Both numeral flags and substitutes can be found in today's naval flag alphabet.

The British Board of Trade created the first iteration of the modern International Code of Signals in 1855. However, the Code would not be used worldwide until 1901, following a conference in Washington, DC. The maritime flag alphabet underwent several changes over the next century, particularly as communication issues during both World Wars demonstrated the limitations of the existing code. 

Some changes included prioritizing safety over general communication, and replacing the method of spelling out messages letter-by-letter with set meanings. The modern signal flags alphabet we use today was last updated in 2005 by the International Maritime Organization.

From their early use as simple commands to the structured International Code of Signals we rely on today, the nautical flags alphabet helps keep ships and their crews safe by bridging language barriers and radio issues. Understanding signal flags and their use offers a fascinating look into how ships worldwide communicate on the open water.