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Since there were no general rules some organizations developed their own distress signals. The first signal was CQD which was approved on January 7, 1904 by "Circular 57" of the Marconi International Marine Communication Company and it was intended for ships equipped with its radios. It was a general call signal for all CQ stations with addition of “D” letter for “danger”. Sailors quickly picked up the phrase "Come Quick, Danger" for the combination of CQD.
But since this signal was used only on ships equipped with Marconi Co. radios it could not be called a single international distress signal.
In 1906 the second International Radiotelegraph Conference took place in Berlin. Representatives of twenty-nine countries took part in its work. The agenda was to establish a unified radiotelegraph distress signal. Representative of Marconi Co. insisted on CQD signal approval. But US representatives didn’t accept it arguing that when transmitting and receiving such a signal it was often confused with the general CQ call. It was decided to replace CQD with some other clear and distinct signal.
A representative of the German Slaby-Arco radiotelegraph company suggested SOE signal. During the discussion one of the delegates noted that the proposed signal has a significant drawback since “E” letter was transmitted by one point only, the signal at a distant reception could be distorted and even not understood. Therefore it was decided to replace “E” with “S”. Thus it turned to be SOS.