SOS

SOS, when it was first agreed upon by the International Radio Telegraphic Convention in 1906, was merely a distinctive Morse code sequence and was initially not an abbreviation.

[2] Moreover, due to its high-profile use in emergencies, the phrase "SOS" has entered general usage to informally indicate a crisis or the need for action.

It became a worldwide standard when it was included in the service regulations of the first International Radiotelegraph Convention signed on 3 November 1906, which became effective on 1 July 1908.

[4] Other prefixes are used for mechanical breakdowns, requests for medical assistance, and a relayed distress signal originally sent by another station.

Radio (initially known as "wireless telegraphy") was developed in the late 1890s, and was quickly recognized as an important aid to maritime communication.

In 1903, an Italian representative at the Berlin Preliminary Conference on Wireless Telegraphy, Captain Quintino Bonomo, discussed the need for common operating procedures, including the suggestion that "ships in distress ... should send the signal SSS DDD at intervals of a few minutes".

The convention adopted an extensive collection of Service Regulations, including Article XVI, which read: "Ships in distress shall use the following signal:  ▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄  repeated at brief intervals".

"[15] (In American Morse code, which was used by many coastal ships in the United States through the first part of the twentieth century, three dashes stood for the numeral "5", so in a few cases the distress signal was informally referred to as "S 5 S".

)[16] The first ships that have been reported to have transmitted an SOS distress call were the Cunard oceanliner RMS Slavonia on 10 June 1909 while sailing the Azores,[17][18] and the steamer SS Arapahoe on 11 August 1909 while off the North Carolina coast.

During World War II, additional codes were employed to include immediate details about attacks by enemy vessels, especially in the Battle of the Atlantic.

However, ships radio room clocks typically had markings on the dial to guide operators in sending the signal manually.

The phrase "SOS" used on a Belgian emergency telephone traffic sign
SOS sent from a flashlight
SOS was introduced for emergency maritime radio communication using Morse code .
Cunard liner RMS Slavonia photographed the day it was wrecked on 10 June 1909; it is the earliest-reported ship to have transmitted the SOS distress call.
Ship's radio room clock
Ship's radio room clock, with 4-second-long red bands with 1-second white gaps around the circumference, so the CW alarm signal could be sent manually. The red and green wedges denote compulsory 3-minute silent periods for receiving weak distress signals.