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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 is a picture of a book showing a happy mix of half-width and full-width.

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6 Answers

up vote 24 down vote accepted

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,

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:

|2|
|0|
|0|
|9|
|月|
|6|
|月|
|30|
|日|

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 (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, which is based on a standard established by Japanese Industrial Standards Committee.
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hmm, that's a very complete answer. good –  repecmps Jul 1 '11 at 14:01
    
Awesome examples! –  William Jul 1 '11 at 17:10
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I don't really think it matters. And even in your image, the full-width numeral is used only to take up the same amount of space as the pair of half-width numerals next to it.

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Although it looks odd if you mix it, but I don't think there is rules for that.

But some web forms only allow 半角 on phone numbers, and some only allow 全角 for Address, so you may need to force one on those places.

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+1 for mentioning webforms. Though I think it is more of a convention or lack of techinical skill cause it shows up in the DB the same. –  Mark Hosang Jun 7 '11 at 9:32
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No, it's a lack of writing conversions; fullwidth digits are distinct unicode codepoints from halfwidth digits, and without any conversion typically fullwidth digits cannot be recognized as such. –  bdonlan Jun 8 '11 at 0:17
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+1 I'm actually fairly certain full-width alphanumerics exist mostly for one reason on the web: to provide another reason for webforms to fail to submit. There has to be some kami, the Japanese Loki, who delights in the suffering of Japanese web users. In his mischief, he litters webforms with demands about using half-/full-width and ex/including hyphens, a zero before the month/day, 丁目, 番地, etc., as though the conversion were something fierce. –  brymck Jul 2 '11 at 8:27
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I always use half width for numbers and letters for purposes of compatibility. Not everything can handle double-byte characters.

As for a matter of style, I like to use something crazy like the numbers in circles (①, ②, ③...) or the old style kanji (壱、弐、参...) for effect (if I'm trying to draw attention, like on ニコニコ動画 or something).

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Agree on the compatibility, especially with programs developed outside of Japan. I encounter programs all the time that don't treat half-width and full-width as the same character. And that's if they even recognize the full-width character in the first place. –  Troyen Jun 8 '11 at 0:30
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You are correct, they don't. The vast majority of (Windows-) programs is still written with extended ASCII, and not Unicode. By the way, most Japanese programs are not written with proper Unicode in mind either, and don't run on a western system without some advanced magics (AppLocale). –  Kdansky Jun 8 '11 at 7:32
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I am not sure if there is any rule either (sounds like too recent a problem to have a strong tradition attached to it), but I thought this comment made (on the original Stack Overflow) by a native Japanese was interesting:

I'm a Japanese, and I loathe full width numerals! Please just use the half-width numerals within Japanese sentences. All you need is to pad them with spaces. ここに10個林檎があります looks bad, but ここに 10 個林檎があります looks completely OK! ここに10個林檎があります looks horrible to me

(funnily enough, as a non-native, my visual preference would be the exact opposite...)

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I am quite surprised that he suggests inserting half-width spaces. Has anyone ever seen a website that uses this convention? –  Nicolas Raoul Jun 13 '11 at 10:09
    
@Nicolas: agreed. As I said: my preference would run the exact opposite (spaces last). Not to mention: the use of spaces might royally screw up word wrap. His might be a fringe viewpoint, but it's still worth taking into consideration, especially since it comes from a somewhat "power-user" who likely has given the matter some thought. –  Dave Jun 13 '11 at 10:16
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Inserting spaces at the boundaries of Japanese and Alpha-numeric is a well established convention, and is often formally taught. –  sawa Jul 2 '11 at 16:13
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You should use only the standard digits "0123456789" (which you call "half-width"), not the full-width forms.

(The digits "0123456789" are not necessarily half the width of a Chinese character, though they typically are in Japanese fonts.)

The full-width variants are considered compatibility characters in Unicode:

Compatibility Character. A character that would not have been encoded except for compatibility and round-trip convertibility with other standards.

Basically, these exist only for compatibility with legacy software, and their use is discouraged. But since they remain on Japanese keyboard layouts, people keep using them.

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