Arabic numbers: half-width or full-width?


Using my computer, I type a lot of numbers in Japanese text, and I am not sure
when to use half-width or full-width.

Are there rules?

Right now I only use half-width, is it fine?

_Off-topic:[Here](https://i.stack.imgur.com/N1g2R.png) is a picture of a book
showing a happy mix of half-width and full-width._




It's perfectly fine to use only half-width arabic numbers.

  * 2009年6月30日

However, there are other rules in operation, coming from various time in the
history of writing and printing:

  * A. Don't use arabic numbers at all - maybe seen in formal documents: 
    * 二千九年六月三十日
  * B. Half-width for two-digit numbers, otherwise full-width - mostly in printed materials: 
    * ２００９年６月30日
  * C. Full-width for a digit, half-width for more than two digits - sometimes used in electronic texts and printed materials: 
    * 2009年６月30日

If you're writing some serious documents, check if there is any style guide
provided that specifies which to use when. Or you can adopt one of the
publicly available style guides. For example,

  * Microsoft's [Japanese style guide](http://www.microsoft.com/Language/ja-jp/StyleGuides.aspx) (chm): 3.2.1 算用数字と漢数字 (Arabic and Chinese Numerals)
  * Sun's [Japanese style guide](http://developers.sun.com/global/technology/translation/style-guides/JA-rule.pdf) (pdf) p.40 半角文字の使い方 (How to use half-width characters)
  * Wikipedia's style guide: [Typography conventions](http://ja.wikipedia.org/wiki/Wikipedia%3a%E8%A1%A8%E8%A8%98%E3%82%AC%E3%82%A4%E3%83%89#.E5.85.B7.E4.BD.93.E4.BE.8B.E3.81.AB.E3.82.88.E3.82.8B.E8.AA.AC.E6.98.8E), part of [Manual of Style](http://ja.wikipedia.org/wiki/Wikipedia%3a%E3%82%B9%E3%82%BF%E3%82%A4%E3%83%AB%E3%83%9E%E3%83%8B%E3%83%A5%E3%82%A2%E3%83%AB)

All of the three recommends to use half-width numbers in general. So there you
have a justification to use half-width numbers only.

[Note: my knowledge is biased towards software documentations]

Historically:

Traditional Japanese typesetting faced the problem of how to arrange
multilingual text that include Japanese and Roman characters in an
aesthetically pleasing way, when Roman characters were introduced to Japan.

Take vertical typesetting. When you're writing vertically, there is no way for
a word with more than two half-width letters to fit in a single cell without
losing readability. They'll have to occupy individual cells vertically next to
each other. In fact, Rule B is actually optimized for vertical writing. ex:

[code]

    |２|
    |０|
    |０|
    |９|
    |月|
    |６|
    |月|
    |30|
    |日|
    
[/code]

When digital publishing entered the scene, these conventions have been carried
over to electronic text composition. Consequently, some people use different
standards for full-width/half-width numerals as we've seen above.

Side note: Another practice in writing electronic text tries to replicate the
aesthetics of horizontal typesetting. That is, inserting a half-width space
whenever there's a pair of half-width letter and full-width letter: `空白を 1 つ`
\- `空白を1つ`. This is a workaround around the fact that modern consumer
softwares don't layout Japanese texts properly. Probably except TeX. Whether
this practice is acceptable or not is a favorite topic of Japanese blogosphere
that pops up every so often.

Further reading:

  * [数字の扱い方](http://www.informe.co.jp/useful/character/character21.html) (how to handle numerals) from a company specializing in desktop publishing. Covers historical backgrounds.
  * More ways of mixing different character types are covered in [W3C's Requirements for Japanese Text Layout](http://www.w3.org/TR/2009/NOTE-jlreq-20090604/#en-subheading2_2), which is based on a standard established by Japanese Industrial Standards Committee.




It's perfectly fine to use only half-width arabic numbers.

  * 2009年6月30日

However, there are other rules in operation, coming from various time in the
history of writing and printing:

  * A. Don't use arabic numbers at all - maybe seen in formal documents: 
    * 二千九年六月三十日
  * B. Half-width for two-digit numbers, otherwise full-width - mostly in printed materials: 
    * ２００９年６月30日
  * C. Full-width for a digit, half-width for more than two digits - sometimes used in electronic texts and printed materials: 
    * 2009年６月30日

If you're writing some serious documents, check if there is any style guide
provided that specifies which to use when. Or you can adopt one of the
publicly available style guides. For example,

  * Microsoft's [Japanese style guide](http://www.microsoft.com/Language/ja-jp/StyleGuides.aspx) (chm): 3.2.1 算用数字と漢数字 (Arabic and Chinese Numerals)
  * Sun's [Japanese style guide](http://developers.sun.com/global/technology/translation/style-guides/JA-rule.pdf) (pdf) p.40 半角文字の使い方 (How to use half-width characters)
  * Wikipedia's style guide: [Typography conventions](http://ja.wikipedia.org/wiki/Wikipedia%3a%E8%A1%A8%E8%A8%98%E3%82%AC%E3%82%A4%E3%83%89#.E5.85.B7.E4.BD.93.E4.BE.8B.E3.81.AB.E3.82.88.E3.82.8B.E8.AA.AC.E6.98.8E), part of [Manual of Style](http://ja.wikipedia.org/wiki/Wikipedia%3a%E3%82%B9%E3%82%BF%E3%82%A4%E3%83%AB%E3%83%9E%E3%83%8B%E3%83%A5%E3%82%A2%E3%83%AB)

All of the three recommends to use half-width numbers in general. So there you
have a justification to use half-width numbers only.

[Note: my knowledge is biased towards software documentations]

Historically:

Traditional Japanese typesetting faced the problem of how to arrange
multilingual text that include Japanese and Roman characters in an
aesthetically pleasing way, when Roman characters were introduced to Japan.

Take vertical typesetting. When you're writing vertically, there is no way for
a word with more than two half-width letters to fit in a single cell without
losing readability. They'll have to occupy individual cells vertically next to
each other. In fact, Rule B is actually optimized for vertical writing. ex:

[code]

    |２|
    |０|
    |０|
    |９|
    |月|
    |６|
    |月|
    |30|
    |日|
    
[/code]

When digital publishing entered the scene, these conventions have been carried
over to electronic text composition. Consequently, some people use different
standards for full-width/half-width numerals as we've seen above.

Side note: Another practice in writing electronic text tries to replicate the
aesthetics of horizontal typesetting. That is, inserting a half-width space
whenever there's a pair of half-width letter and full-width letter: `空白を 1 つ`
\- `空白を1つ`. This is a workaround around the fact that modern consumer
softwares don't layout Japanese texts properly. Probably except TeX. Whether
this practice is acceptable or not is a favorite topic of Japanese blogosphere
that pops up every so often.

Further reading:

  * [数字の扱い方](http://www.informe.co.jp/useful/character/character21.html) (how to handle numerals) from a company specializing in desktop publishing. Covers historical backgrounds.
  * More ways of mixing different character types are covered in [W3C's Requirements for Japanese Text Layout](http://www.w3.org/TR/2009/NOTE-jlreq-20090604/#en-subheading2_2), which is based on a standard established by Japanese Industrial Standards Committee.

