O in Morse code

O is three steady tones of the same length. At 18 WPM each dash lasts 200 ms, with a 66.7 ms silence between them, about 733 ms for the letter. There is no dot to break the rhythm, so the count is everything.

• Say it:

  dah-dah-dah, three equal heavy beats.

• Tap it:

  three long presses with short pauses between.

• Flash it:

  three long blinks of a light.

• Write it:

  three hyphens together, then a space.

Two short words show O in use. NO is -. --- in Morse code, 19 units long, and YOU is -.-- --- ..- at 37 units, O in the middle. Type any other word into the Morse code translator to hear it.

In a memory phrase every stressed syllable is a dash. O has three dashes and no dots, so its phrases are three strong words in a row: “OH MY GOD”, “HO HO HO” or “ONE OF US”.

Douglas Harder's written mnemonic, which uses tall letters for dashes, is OPP: three capitals for three dashes. The Morse code mnemonics guide has a phrase for every letter.

• M:

  one dash shorter, -- with two long tones.

• The number 0:

  two dashes added, so ----- is five long tones.

• S:

  every dash turned into a dot, so ... is O's opposite.

• G:

  the last dash shortened, so --. ends on a dot.

Turn any one dash into a dot and O becomes W, K or G, depending on which dash you change. O reads the same backward, so there is no reversed form to learn. To practice the all-dash run T, M, O and 0, use the Sound-alike drill.

The distress signal SOS is three dots, three dashes and three dots, the dashes being O. It is sent as one continuous signal, ...---... in Morse code, with no letter gaps between the S, O and S. That makes it 27 units, about 1.8 seconds at 18 WPM.

The SOS page plays it at a speed you can follow, and How to signal SOS covers sending it with a light, a whistle or a mirror.

The code gave short patterns to common letters: Alfred Vail estimated how often each letter is used by counting the type in a newspaper's type cases. O breaks that rule. It is the fourth most common letter in English, about 7.5% of letters in Robert Lewand's count, yet at 11 units it is the longest of the ten most common letters.

American Morse, the US landline code, gave O a short form: two dots with a longer space between them, . . in writing, which an International operator would read as EE. International Morse took three dashes instead. Compare O with every other letter on the alphabet chart.