250px|thumb|right|class=skin-invert-image|Display of zero in three typefaces, from top to bottom: slashed zero, dotted zero, plain or open zero

A slashed zero is a representation of the Arabic digit zero ("0") with a slash through it. This variant zero glyph is often used to distinguish the digit zero from the Latin script letter O anywhere that the distinction needs emphasis, particularly in encoding systems, scientific and engineering applications, computer programming (such as software development), and telecommunications. It thus helps to differentiate characters that would otherwise be homoglyphs. It was commonly used during the punch card era, when programs were typically written out by hand, to avoid ambiguity when the character was later typed on a card punch.

Usage

thumb|right|Slashed zeroes on a bus stop sign in Portugal, 2020

The slashed zero is used in a number of fields in order to avoid confusion with the letter "O". It is used by computer programmers, in recording amateur radio call signs and in military radio, as logs of such contacts tend to contain both letters and numerals.

The slashed zero was used on teleprinter circuits for weather applications. In this usage it was sometimes called communications zero.

The slashed zero can be used in stoichiometry to avoid confusion with the symbol for oxygen (capital O).

The slashed zero is also used in charting and documenting in the medical and healthcare fields to avoid confusion with the letter "O". It also denotes an absence of something (similar to the usage of an "empty set" character), such as a sign or a symptom.

Slashed zeros are used on New Zealand number plates.

History

The slashed zero predates computers, and is known to have been used in the twelfth and thirteenth centuries.

In the days of the typewriter, there was no key for the slashed zero. Typists could generate it by first typing either an uppercase "O" or a zero and then backspace, followed by typing the slash key.

It is used in many Baudot teleprinter applications, specifically the keytop and typepallet that combines "P" and slashed zero. on this browser it produces .

Note that the above should not be confused with the "slashed zero variant of the empty set", <math>\emptyset</math>, as popularized by Donald Knuth's TeX. Unicode represents that character as the empty set (&emptyset;) with variation selector 1.

In HTML, slashed zero can be enabled by using CSS property <syntaxhighlight lang=css inline>font-variant-numeric: slashed-zero</syntaxhighlight> or alternatively <syntaxhighlight lang=css inline>font-feature-settings: 'zero'</syntaxhighlight>. On this browser, this renders as <span style="font-variant-numeric: slashed-zero"></span>.

Typography

German license plate depicting diagonal gap|thumb|right

In most typographic designs, the slash of a slashed zero usually does not extend past the ellipse. This contrasts with the Scandinavian vowel "Ø", the empty set symbol "∅", and the diameter symbol "⌀". Conversely, Japanese typefaces frequently render slashed zero with the slash extending beyond the ellipse.

A convention common on early line printers left zero unornamented but added a tail or hook to the letter-O so that it resembled an inverted Q (like U+213A ℺) or cursive capital letter-O (<math>\,\mathcal{O} \,</math>).

  • Andalé Mono has a dotted zero.
  • IBM Plex Mono uses a dotted zero.
  • Source Code Pro and its associated typefaces use a dotted zero.
  • Cascadia Code, the default font for Windows Terminal, Visual Studio, and Visual Studio Code, uses a dotted zero.

Variations

Dotted zero

The zero with a dot in the center seems to have originated as an option on IBM 3270 display controllers. The dotted zero may appear similar to the Greek letter theta (particularly capital theta, Θ), but the two have different glyphs. In raster fonts, the theta usually has a horizontal line connecting, or nearly touching, the sides of an O, while the dotted zero simply has a dot in the middle. However, on a low-definition display, such a form can be confused with a numeral 8. In some fonts the IPA letter for a bilabial click (ʘ) looks similar to the dotted zero.

Alternatively, the dot can become a vertical trace—for example, by adding a "combining short vertical line overlay" <code>(U+20D3)</code>. It may be coded as <code><nowiki>0&amp;#x20D3;</nowiki></code> giving <span style="font-family:Code2000" class="Unicode">0⃓</span>.

The dotted zero has been used on the vehicle registration plates of Slovakia since 2023.

Slashed letter 'O'

thumb|[[Apollo 11 video display terminal with a slashed O]]

IBM (and a few other early mainframe makers) used a convention in which the letter O had a slash and the digit 0 did not.

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Sources

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