Whether you declare “SOS” in Morse code or spell it out in seashells on a desert island, a vast majority of the world will understand that you’re in need of help. But before “SOS” was the international distress symbol, “CQD” did the job.
The signal “CQD” was derived from an earlier code, “CQ,” commonly used by telegraphers and wireless operators to address all stations at once. It was so common, in fact, that it became overused and lost the sense of urgency it was meant to convey.
As the Marconi Company became the leader in wireless telegraphy in the early 1900s, they decided a new signal was needed. They kept “CQ” for its familiarity but modified it with the extra “D” to denote distress. Though some have retroactively applied the phrase “Come Quick Danger” to the letters, Marconi himself once said that the letters weren’t meant to be an acronym: “It [CQD] is a conventional signal which was introduced originally by my company to express a state of danger or peril of a ship that sends it."
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Despite Marconi’s push for “CQD,” not all nations were on board. The British used it, but Americans kept “NC,” which meant “call for help without delay.” Meanwhile, the Germans used “SOE,” while Italians liked the unmistakable “SSSDDD.”
By 1906, delegates at the second International Radio Telegraphic Conference realized that an international signal was desperately needed, and proposed “SOS” for its ease of transmission; the pattern ”...---...” in Morse code was simple and immediately recognizable. It was officially ratified by all conference members by 1908—except for the United States, which took a bit longer to adopt the practice.
Still, it took some time for “CQD” to leave the vernacular. In fact, the night the Titanic went down in 1912, the wireless operators were still using it. They also tried “SOS” after junior wireless operator Harold Bride joked to senior operator Jack Phillips that it might be his last chance to use the new distress call. Sadly, it was—Phillips went down with the ship. Not long after that, the U.S. adopted "SOS" as its official distress signal.
Though “CQD” is long gone, “CQ” is still popular with ham radio operators—and it’s still used to establish contact, just as British operators used it more than a century ago.