SOS, code letters of the international distress signal. The signal is expressed in International Morse code as … — — — … (three dots, three dashes, three dots). This combination was established by the International Radiotelegraphic Convention at Berlin in 1906. The letters (SOS) do not refer to any words but were selected because they are easy to transmit. The use of Morse code for sending distress calls is now superseded by automated systems using satellite relay; the U.S. Coast Guard no longer monitors Morse code transmissions. The distress code by radiotelephony is MAY DAY, which corresponds to the French "m'aider." The signal NC, not followed by a message, also has the same meaning.