The International Code of Signals (ICS) is an international system of signals and codes for use by vessels to communicate important messages regarding safety of navigation and related matters. Signals can be sent by flaghoist, signal lamp ("blinker"), flag semaphore, radiotelegraphy, and radiotelephony. The International Code is the most recent evolution of a wide variety of maritime flag signalling systems.

Prior to 1969, the code was much more extensive, covering a wider range of messages and including a list of five-letter codes for every prominent maritime location in the world. Since 1969, it has been reduced to focus on navigation and safety, including a medical section. Signals can be sorted into three groups:

* Single-letter signals which are very urgent, important, or common.

* Two-letter signals for other messages, sometimes followed with a numerical "complement" which supplements or modifies the message.

* Three-letter signals beginning with "M"; these are the Medical Signal Codes.

In the single-letter signals (using one flag), Oscar is used to indicate "Man overboard."

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