Slashed zero
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The slashed zero looks just like a regular letter 'O' or number '0' (zero), but it has a slash through it. Unlike the Scandinavian vowel 'Ø' and the "empty set" symbol '∅', the slash often touches the walls of the surrounding O shape but does not extend past them on the outside.
It is used as the glyph for the number 0 on character displays in mainframe and some personal computers to distinguish the letter 'O' from the number '0'.
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[edit] Origins
The slashed zero, looking identical to the letter O other than the slash, is used in many ASCII graphic sets descended from the default typewheel on the Model 33 Teletype.
The slashed zero long predates computers, and has been known to have been used in the twelfth and thirteenth centuries (Cajori, 1928).
[edit] Usage
When computers first 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.
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 article "Heavy metal umlaut").
The slashed zero symbol is widely used in written Amateur radio callsigns, New Zealand alphanumeric car number plates, codes for video-games, software product keys, and any other instance when clarity is necessary.
Many people slash the zeroes on cheques in order to prevent fraud, for example: changing a 0 to an 8.
[edit] Representation in Unicode and HTML
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 (Internet Explorer fails to render them).
They may be coded as 0̸ giving 0̸̸
[edit] Similar symbols
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 letter Phi in some fonts (although usually, the slash is vertical).
- The symbol "∅" (U+2205) is used in mathematics to refer to the empty set.
- "⌀" (U+2300) is used as the standard symbol for diameter, though the official symbol is slightly stylised (the stroke is often thinner at the bottom and thicker at the top, like the club or baton shape of the exclamation point; and extends further above the o portion).
- 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.
[edit] Variations
[edit] Dotted zero
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), 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.
[edit] Slashed 'O'
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.
[edit] Reversed slash
Some Burroughs/Unisys equipment displays a zero with a reversed slash, as 
[edit] Other
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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, there is a diagonal gap on the top right of the zero.
[edit] References
Cajori, Florian (1928-1929). A History of Mathematical Notations. Chicago: Open Court Pub. Co. ISBN 0-486-67766-4.
