The slashed zero is a representation of the number "0" (zero), with a slash through it. 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.
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". Some fonts make slashed zero look like "0/".
In character encoding terms, it has no explicit Unicode code point, but "short diagonal stroke form" of digit zero is defined as a variation sequence using Variation Selector-1 (U+FE00); i.e., the sequence U+0030 is immediately followed by U+FE00.
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. Additionally, the slashed zero is used in many ASCII graphic sets descended from the default typewheel on the Teletype Model 33.
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, sometimes called communications zero, was used on teleprinter circuits for weather applications.
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.
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.
To generate a slashed zero on typewriters, typists would type a normal "O" or zero, backspace, and then 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'.
Slashed zeroes can also be used on cheques in order to prevent fraud, for example: changing a 0 to an 8.
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.
Unicode supports explicit slashed zero, but only via a pair of combining characters, not as a distinct single character (or code point, 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. They may be coded as
0̸ giving 0̸.
The slashed zero has the disadvantage that it can be confused with several other symbols:
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"
(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 had a slash and the digit 0 did 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.
This was later flipped and most mainframe chain or band printers used the opposite convention (letter O printed as is, and digit zero printed with a slash Ø). This was the de facto standard from 1970s to 1990s. However current use of network laser printers that use PC style fonts caused the demise of the slashed zero in most companies - only a few configured laser printers to use Ø.
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 (like U+213A ℺) or cursive capital letter-O ().
In the Fixedsys typeface, the numeral 0 has two internal barbs along the lines of the slash. This appears much like a white "S" within the black borders of the zero.
Dotted zero typefaces: