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 self-same 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 (except in hasty handwriting).
The slashed zero glyph is often used to distinguish the digit "zero" ("0") from the Latin script letter "O" anywhere that the distinction is important, particularly in encoding systems, scientific and engineering applications, computer programming (such as software development), and telecommunications.
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The slashed zero long predates computers, and has been known to have been used in the twelfth and thirteenth centuries.[1]
The Slashed zero is used in many Baudot teleprinter applications, specifically the keytop and typepallet that combines "P" and slashed zero.[2]
The slashed zero is used in many ASCII graphic sets descended from the default typewheel on the Teletype Model 33.
The use of the Scandinavian vowel ø in the name of the Hawkwind-influenced 1980s space-rock band Underground Zerø may have been inspired by the usage of the slashed zero by many computer systems of the time; which resembled ø (see “Heavy metal umlaut”).
The slashed zero, sometimes called communications zero, was used on teleprinter circuits for weather applications.[3]
The slashed zero symbol is widely used in written Amateur radio callsigns, codes for video-games, software product keys, and any other instance when clarity is necessary.
When personal computers started to become mainstream in the early 1980s, it became one of the things associated with the hacker culture of the time. 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.
The slashed zero is supported, but 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, giving the effect of the slashed zero.
When used in HTML, use of these 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:
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 is the most common variation today. It seems to have originated as an option on IBM 3270 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 similar.
The slashed zero is also common in recording amateur radio call signs for logs of contacts to distinguish a zero (i.e. Ø) from a capital 'O'.
Some Burroughs/Unisys equipment displays a zero with a reversed slash, as ⃠.
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.
There are very few typefaces commonly included in a PC that use the slashed zero. They include: