[This is a fast-access FAQ excerpt.]
SOS does NOT stand for "Save Our Ship/Souls", for "Stop Other
Signals", for "Send Our Saviour/Succour", for "Sure of Sinking", or
for the Russian Spasiti Ot Smerti (= "save from death"). The
signal "...---...", recommended for international distress calls at
the international Radio Telegraph Conference of 1906 and officially
adopted in 1908, was not chosen for any alphabetic significance.
Such a signal is now known as a "prosign" (from "procedural
signal"). Those prosigns (such as this one) that are transmitted
without interletter gaps are notated with an overbar. Since
"..." is S and "---" is O in Morse code, the distress signal is
conventionally represented as:
___
SOS
but since there are no interletter gaps, it could also be analysed
as various other combinations of Morse code letters.
Fred Bland writes: "Three of anything (e.g. gun shots, fires,
cairns) is a conventional signal of distress recommended in survival
guides. I don't know whether this convention or the use of three
dots and dashes is older."
Mark Brader writes: "The sign used before SOS was CQD, which
was composed of the usual 'calling' sign CQ, plus D for Distress.
Even in 1912 when the Titanic was sinking, its operator put out a
CQD first and only added SOS after being reminded."
Thomas Hamilton White (whitetho@med.unc.edu) writes: "I have
read that the international distress call evolved from SOE (sent as
three letters), which had been used as a distress signal by German
companies. However, because the final E in this sequence consisted
of a single dot, the signal was modified to ...---... to be more
distinctive and symmetrical. [...] I can think of one very practical
reason for continuing to informally treat the distress signal as
SOS -- ever try to stamp ...---... in a snowbank?"
[See some additional "SOS" comments.]