"SOS"

     [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.]