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I was working on the Morse code Wikipedia article , when I noticed something strange.
There might be a reason, but I can't find it.

As you can see, the code symbol ...-. is not used, however the code symbol ----- is. Why?

The first symbol is shorter (11) than the second one (19).
This is not the only code symbol unused.

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Three possible reasons:

  1. ...-. is used by a prosign meaning understood;
  2. using ----- for the digit 0 makes the encoding of digits regular, thus easier to memorize;
  3. ----- for the digit 0 could be replaced by the shorter --- for the letter O, in contexts where it is clear that a digit is thought.

I conjecture that 2 is the historical reason.

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    $\begingroup$ I have been CW (Morse Code) operator for many years (decades) and I have never heard the digit zero sent as the letter O. But, alternatively, the digit zero is often sent as a long dash and I do this myself. Hard to do with a paddle and keyer and because of that I have a straight key available at all times. The digit 9 (----.) is also a long code and it is often sent as the letter N but usually only in a signal report of 5NN sent instead of 599. $\endgroup$
    – K7PEH
    Jan 29, 2016 at 15:52
  • $\begingroup$ @K7PEH: This question was recently bumped to the "front page" (an edit of vonbrand's answer) and after seeing your comment I thought I'd mention that my father got his Extra class license in 1951 (age 16 or 17) and was a "high-speed coder", one of two or three dozen people in the world (at least in the late 1960s to early 1980s) who communicated 50-70 WPM (I think) in special a (some?) dedicated band location(s). I never, ever recall him writing anything down. (continued) $\endgroup$
    – Dave L Renfro
    Oct 2, 2021 at 20:44
  • $\begingroup$ Often when he was finished sending (using a special device that you touch left-and-right, not an up-and-down key) he'd go get something from the kitchen, listening to the code as one would listen to someone talk (sound always turned up a bit high for my taste). Many times when doing this he'd talk to my mother, something like: "I'm talking to Carl in New Mexico now. Oh, just now he said that his dog recently died and . . . Oh, now he wants to know how you're doing. What should I tell him?" Being so saturated with this growing up, I stayed away from anything electronics (continued) $\endgroup$
    – Dave L Renfro
    Oct 2, 2021 at 20:51
  • $\begingroup$ (which probably also has to do with my neglect of computers -- took a PL-1 programming class in 1977, nothing since to the determinant to my financial situation as pure math doesn't really get you anywhere except teaching unless you're really, really good at it), but my 2-years younger brother is a ham and does electronics repair work, mostly cell-phone towers stuff I think. $\endgroup$
    – Dave L Renfro
    Oct 2, 2021 at 20:55
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Morse code was defined by Morse himself, and later (somewhat) extended. Using letter frequency for dense coding was a consideration (. for the commonest letter e, and on down), trying to make it easy to learn clearly was a goal, and (some) resistance to garbling; but all this happened long before Shannon's study of communication or any systematic study of coding, error detection and such.

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