The International Code of Signals descends from the first code issued by the British Board of Trade in 1855. The current version was approved by the International Maritime Organization (IMO) in 1965 and has been adopted by all maritime nations.

Its main function is to resolve situations related to the safety of navigation and people and is based on signal flags used in maritime navigation to transmit messages, either between two or more ships, or between a ship and land or the port.

The flags, with different shapes and colors, each represent a letter of the international alphabet, as well as the numbers from 0 to 9. Each letter or group of letters of the alphabet, represented by flags, has a particular meaning.

Because of its shape, a flag is called a block when it is rectangular, a cornet if it ends in two points on the side opposite the sheath, and a pennant if it is triangular.

It is formed by twenty-six alphabetical flags (two of them cornets and the rest blocks), ten numerical pennants, three repetitive pennants and a characteristic pennant.

When a ship needs to transmit a message consisting of one or more words, or numbers, hoist the flags representing the letters and numbers of the message on the front mast, aligned from top to bottom. If the message is longer, the operation will be repeated with new flags.
Flags are also used individually or in combinations of two, with a determined meaning according to an internationally valid code.

As some messages can also be transmitted by means of light signals or by radiotelephony, the Code is no longer commonly used.

On the other hand, the individual flags or in combinations of two are still widely used to signal a given warning.