From Daryl Stout@HURRICAN/THUNDER to All on Sat Mar 4 10:15:00 2017
Many of the expressions and procedure signals still in use in
radiotelegraph had their origins in the early days of the landline telegraph--long before Marconi sent his letter "S" across the Atlantic.
In sending formal messages by c.w., the first thing a beginner hears is
"don't send punctuation. Separate the parts of the address from each
other with the prosign AA." This is ironic, because in the American Morse
Code the sound didahdidah is a comma, and was doubtless the origin of our prosign. Originally, a correctly addressed letter was punctuated with
commas following the name and the street address, each of which was
(and still is) on a separate line although the commas have been dropped,
even in mail addresses on letters. The comma was transmitted by Morse
operators and thus, AA came to mean that the receiving operator should
"drop down one line" when sent after each part of the address, and it is
so defined in the operating manuals of the time.
Our familiar prosign SK also had its origin in landline Morse. In the
Western Union company's "92 code" used even before the American Civil
War, the number 30 meant "the end. No more." It also meant "good night."
It so happens that in Landline Morse, 30 is sent didididahdit daaah, the
zero being a long dash. Run the 30 together and it has the same sound as