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The ITU's International Morse Code recommendation ITU-R M.1677-1 specifies a "Full stop (period)" as ".−.−.−", however it falls short of specifying a decimal point. Even the examples, though they illustrate fractional numbers, don't show decimal usage.

ITU's Phonetic Alphabet and Figures regulations (AP14), however, define a "Decimal point" and require it to be spoken as "DAY-SEE-MAL".

What then is the decimal in Morse code, and where is it defined?

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3 Answers 3

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In the International Code of Signals (an International system of maritime communication, including radiotelegraphy), AAA (over-bar) is designated for both full stop and for decimal point. (See flashing light procedure signaling, page 20.)

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    $\begingroup$ And this makes complete sense, because the decimal and period (aka full stop) have been the same on typewriter keyboards since the invention of the machine (not long after the telegraph), and in printing since the advent of hand-set type (17th century). You distinguish between them by context, just as with handwriting or in print. $\endgroup$
    – Zeiss Ikon
    Aug 31, 2019 at 16:15
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    $\begingroup$ From wikipedia searches: International Morse code was proposed circa 1848 (Hamburg) and codified Internationally circa 1865. The Sholes and Glidden typewriter was first marketed circa 1874. So the code preceded the typewriter keyboard (as we now know it). However: Gutenburg circa 1450. Have to check to see if any early printings included decimal numbers (when were these popularized?). $\endgroup$
    – hotpaw2
    Aug 31, 2019 at 16:22
  • $\begingroup$ Napier (discoverer of the idea of logarithms) is reported to have first used the full stop or period as a numeric decimal point circa 1614 and 1619. $\endgroup$
    – hotpaw2
    Aug 31, 2019 at 16:30
  • $\begingroup$ Okay, wrong on the source, but right on the century for origin of the full stop as decimal divider. $\endgroup$
    – Zeiss Ikon
    Aug 31, 2019 at 16:56
  • $\begingroup$ Can we use this in Amateur Radio? $\endgroup$ Sep 1, 2019 at 0:27
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The web sites of Kent Morse Keys and of the AC6V reference compendium document use of the Morse letter 'R' (dit-dah-dit) as an abbreviation for 'decimal point.' 'R' can have other meanings, of course, but I have used and heard it used to represent a decimal point in numbers - particularly when specifying a frequency - for 50 years.

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    $\begingroup$ Interesting! I wonder if there's any historical explanation/mnemonic there? $\endgroup$ Sep 4, 2019 at 0:12
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The American Radio Relay League, ARRL, in their code practice texts, use "R" for the decimal point. I suspect it stands for the word, "radix" (root) because the mark used to separate whole numbers from fractions in any number system (decimal, binary, etc.) is a "radix point".

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    $\begingroup$ In many countries outside the USA the decimal point is marked with a comma. Using a period would just be wrong. "R" is neutral on that score. $\endgroup$
    – Pete NU9W
    Mar 27, 2023 at 12:54

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