0 in Morse code

Every digit in International Morse code is five elements long. 1 to 5 open with as many dots as their value and finish with dashes; 6 to 9 open with dashes instead, one for 6 up to four for 9. Zero closes the run as ten would: all five places are dashes.

That makes 0 the mirror image of 5, which is ....., five dots with no dash. Learn the pair together and the rest of the set falls between them: 9 is ----., zero with its last dash turned into a dot, and 1 is .----, a single dot in front of four dashes.

A run of dashes means different things depending on its length. One dash is T, two are M (--), three are O (---) and five are 0. Four dashes are not in the ITU table at all.

When you copy by ear, the length of the whole tone train is the clue: 0 lasts 19 units, O only 11.

On paper, radio operators often write zero with a slash through it, as Ø. Call signs mix letters and figures, so a log full of them needs a zero that cannot be read as the letter O.

Since the 1870s operators have used cut numbers, short forms of the digits that only work when both sides expect figures, such as a contest serial number. Zero is cut to a single dash, the letter T, which saves 16 of its 19 units. A serial number 100 sent in full is .---- ----- -----, and cut it goes out as 1TT.

The cut forms of every digit are listed together on the numbers page. Outside a contest or a known number field, send 0 in full so nobody reads it as T.

• Tap it:

  five long presses of equal length, each three times as long as a dot, with a short gap between them. At 18 words per minute each dash lasts 200 ms and the whole 0 takes about 1.27 seconds.

• Flash it:

  as light it takes longer, because the site holds a flashing light to 7 words per minute, so each flash lasts about 514 ms and the whole 0 about 3.3 seconds. The Flashlight signaler keeps that pace for a longer message.

• Write it:

  five hyphens with no space between them, then a space before the next character. Paste what you wrote into the translator to check that it reads back as 0.

Zero also builds a sign the standard does not have. ITU-R M.1677-1 sends the percent sign as 0, the fraction bar and 0 again, so 0/0 is written ----- -..-. -----, with the slash in the middle as its own character.