The code is transmitted either as an audio tone, a steady radio signal switched on and off (known as continuous wave, or CW), an electrical pulse down a telegraph wire, or as a mechanical or visual signal (eg. a flashing light). Two kinds of Morse Code have been used, each with a number of different representations for the symbols used in written English. American Morse Code was used in the wired telegraph systems that made up the first long-distance electronic communication system. "International Morse Code" is most commonly used today.
In addition to the Morse code, elaborate commercial codes that encoded complete phrases into single words (five-letter groups) were developed, so that telegraphers became conversant with such "words" as BYOXO ("Are you trying to crawl out of it?"), LIOUY ("Why do you not answer my question?"), and AYYLU ("Not clearly coded, repeat more clearly."). The purpose of these codes was to save on cable costs.
International Morse Code is still in use today. Since it relies only on a steady (unmodulated) radio signal, it requires less hardware to send and receive than other forms of radio communication, and it can be used in very high noise / low signal situations, and requires very little bandwidth. Until the 1990s (when did the code-free Tech first appear?), the ability to transcribe morse code sent at 5 words per minute was a requirement to receive an FCC Amateur Radio license in the US using HF bands. 20 WPM was required for the license with highest privileges ("Extra" class). Certain parts of the radio spectrum are still(?) reserved only for transmission of morse code signals.
Amateur and military radio operators skilled in morse code can often understand ("copy") code in their heads at rates in excess of 40 WPM.
Timing and representation:
There are two "symbols" used, called dots and dashes or dits and dahs. The length of the dit determines the speed at which the message is sent, and is used as the timing reference. Here is an illustration of the timing conventions. Its intent is to show exact timing - it would normally be written something like this:
-- --- *-* *** * / -*-* --- -** * M O R S E (space) C O D E
where - represents dah and * represents dit. Here's the exact conventional timing for the same message (= represents signal on, . represents signal off, each for the length of a dit):
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===.===...===.===.===...=.===.=...=.=.=...=.......===.=.===.=...===.===.===...===.=.=...=
^ ^ ^ ^ ^
| dah dit | word space
symbol space letter space
A dah is conventionally 3 times as long as a dit. Spacing between dits and dahs in a character is the length of one dit. Spacing between letters in a word is the length of a dah (3 dits). Spacing between words is 7 dits.
People familiar with Morse Code often speak or write it like this. ("Dah" is pronounced with an "awe" vowel sound).
-- --- *-* *** * / -*-* --- -** *
DahDah DahDahDah DiDahDit DiDiDit Dit, DahDiDahDit DahDahDah DahDiDit Dit.
Here's a table including the alphabet and some other commonly used symbols:
| Letter | Code | Letter | Code |
|---|---|---|---|
| A | *- | N | -* |
| B | -*** | O | --- |
| C | -*-* | P | *--* |
| D | -** | Q | --*- |
| E | * | R | *-* |
| F | **-* | S | *** |
| G | --* | T | - |
| H | **** | U | **- |
| I | ** | V | ***- |
| J | *--- | W | *-- |
| K | -*- | X | -**- |
| L | *-** | Y | -*-- |
| M | -- | Z | --** |
The numbers are as follows:
0 ----- 1 *---- 2 **--- 3 ***-- 4 ****- 5 ***** 6 -**** 7 --*** 8 ---** 9 ----*
Common punctuation:
. *-*-*- , --**-- ? **--** - -****- / -**-*
Special characters:
error ******** + *-*-* stop (end of message) @ ***-*- end (end of contact) SOS ***---*** international distress call. Used first time ever by the Titanic on April 15, 1912.
It is a common misunderstanding that the SOS is three symbols but the correct way of sending it is as one morse symbol.
Swedish extensions to the Morse Code:
å *--*- ä *-*- ö ---* ! **--*