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In page CQD:

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This signal soon became known as SOS because it has the same dash-dot sequence as the letters with the gaps between them removed, and in fact is properly written SOS, with an overbar, to distinguish it from the three individual letters[citation needed]. In contrast, CQD is transmitted as three distinct letters with a short gap between each, like regular text. SOS is also easier to hear as it is nine symbols long, while no other character or sign is longer than six symbols. Germany had first adopted SOS in regulations effective 1 April 1905.[2][3]