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The slashed zero is a representation of the number '0' (zero), with a slash through it. In character encoding terms, it is an alternate glyph (in addition to the open zero glyph) for the zero character. Unlike in the Scandinavian vowel 'Ø' and the "empty set" symbol '∅', the slash of a slashed zero usually does not extend past the ellipse in most typographic designs. However the slashed zero is sometimes approximated by overlaying zero and slash characters, producing the character "0̸".
The slashed zero glyph is often used to distinguish the digit "zero" ("0") 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 punched card era, when programs were typically written out by hand, to avoid ambiguity when the character was later typed on a card punch.
The slashed zero long predates computers and is known to have been used in the twelfth and thirteenth centuries. It is used in many Baudot teleprinter applications, specifically the keytop and typepallet that combines "P" and slashed zero. Aditionally, the slashed zero is used in many ASCII graphic sets descended from the default typewheel on the Teletype Model 33.
The slashed zero, sometimes called communications zero, was used on teleprinter circuits for weather applications.
The slashed zero is also common for in logs of contacts to distinguish a zero (i.e. Ø) from a capital 'O', since amateur radio call signs contain both letters and numerals.
Along with the Westminster, MICR and OCR-A fonts, the slashed zero became one of the things associated with hacker culture in the 1980s. Some cartoons depicted computer users talking in binary code with 1s and 0s using a slashed zero for the 0.
Slashed zeroes can also be used on cheques in order to prevent fraud, for example: changing a 0 to an 8.
To generate a slashed zero on typewriters, typists would type a normal zero, backspace, and hit the slash key to mark the zero.
The use of the slashed zero by many computer systems of the 1970s and 1980s inspired the 1980s space rock band Underground Zerø to use a Heavy metal umlaut Scandinavian vowel ø in the band's name and as the band Logo on all their album covers (see link below)
Slashed zeroes have been used in the Flash-based artwork of Young-Hae Chang Heavy Industries, notably in their 2003 work, Operation Nukorea. The reason for their use is unknown, but has been conjectured to be related to themes of 'negation, erasure, and absence'.
Representation in Unicode and HTML
The treatment of slashed zero as a glyph is supported by any font whose designer chose the option. Successful display on any local system depends on having the font available there, either via the system's font files or via font embedding.
The treatment of slashed zero with distinct character encoding is supported by Unicode, but only via a pair of combining characters, not as a distinct single character (or codepoint, in Unicode parlance). It is treated literally as "a zero that is slashed" and it is coded as two characters, the commonplace zero and then the "combining long solidus overlay"
(U+0338). These combining characters overlay the preceding character, creating a composite glyph.
When used in HTML, use of such combining characters is valid but not yet supported by all current web browsers; for instance, Microsoft Internet Explorer fails to render them. They may be coded as
0̸ giving 0̸.
The slashed zero has the disadvantage that it can be confused with several other symbols:
- The slashed zero format causes problems for certain Scandinavian languages — Ø is used as a letter in the Danish, Faroese and Norwegian alphabets, where it represents [ø] or [œ].
- It also resembles the Greek letters Theta and Phi in some fonts (although usually, the slash is horizontal or vertical, respectively).
- The symbols Ø and "∅" (U+2205) are used in mathematics to refer to the empty set.
- "⌀" (U+2300) is Unicode's codepoint for the diameter symbol.
- In German-speaking countries, Ø is also used as a symbol for average value: average in German is Durchschnitt, directly translated as cut-through.
However the unslashed zero has the disadvantage that it is easily confused with the letter 'O'.
In paper writing one may not distinguish the 0 and O at all, or may add a slash across it in order to show the difference, although this sometimes causes ambiguity in regard to the symbol for the empty set.
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.
Alternatively, the dot can become a vertical trace, for example by adding a “combining short vertical line overlay”
(U+20D3). It may be coded as
0⃓ giving 0⃓.
IBM (and a few other early mainframe makers) used a convention in which the letter O has a slash and the digit 0 does not. This is even more problematic for Danes, Faroese, and Norwegians because it means two of their letters—the O and slashed O (Ø)—are visually similar.
Use of the "combining short solidus overlay"
(U+0337) produces a result where the slash is contained within the zero. This may be coded as
0̷ to yield 0̷.
Yet another 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 or cursive capital letter-O. In the Fixedsys typeface, the numeral 0 has two internal barbs along the lines of the slash, which can alternately be considered a narrow “S” within the zero. On German car license plates, which use the FE-Schrift typeface, there is an “insinuated” slash in zeros: a diagonal crack just beneath the top right curvature of the zero.
- Terminal in Microsoft's Windows line.
- Consolas in Microsoft's Windows Vista, Windows 7, Microsoft Office 2007 and Microsoft Visual Studio 2010
- Menlo in OS X
- Monaco in OS X
- The Fedora Linux distribution ships with a tweaked variant of the Liberation typeface which features the use of a slashed zero; this is not present on most existing Linux distributions.
- The DejaVu family of typefaces has a "DejaVu Sans Mono" variant with a dotted zero.
- Andale Mono has a dotted zero.
- ProFont has a slashed zero.
- Cajori 1928.
- "Teletype Printing Telegraph Systems, Keytops and Typepallets", Bulletin 1164B, April 1958: 1-6, Teletype part number 99564(keytop)
- Teletype Parts Bulletin 1184B pages 27 - 29 figure 29 - 31
- "Reference Data For Radio Engineers, Fifth Edition, Howard W. Sams & Co., Inc., 1970: 30-38 Table 23
- Tuck, Michael (2010-08-03). "A Web Designer's Guide to Linux Fonts". Six Revisions.
- Cajori, Florian (1928–29), A History of Mathematical Notations, Chicago, IL: Open Court Pub.; op. cit., New York: Dover Publications, 1993, ISBN 0-486-67766-4.