SOS became the international maritime distress signal (3/10/1906)

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At the First International Radiotelegraph Convention, in Berlin. The Germans had already begun using this signal.
"neither so short as to be ambiguous nor so long as to be unwieldy"
(Crystal worded this with "too", which seems wrong.)
It's technically a _prosign_ (procedural sign) -- a single unit, not a letter sequence.
it's an _ambigram_ -- reads the same when flipped over (useful if you've written it on the ground and people are searching for you from different directions...)