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{{Unreferenced section|date=November 2018}}
[[File:Alternative names of JIS X 0213.svg|thumb|Characters in both [[JIS X 0201]] (single byte) and [[JIS X 0208]] / [[JIS X 0213]] (double byte) have both a halfwidth and a fullwidth form in [[Shift JIS]].]]
In the days of [[text mode]] computing, Western characters were normally laid out in a grid on the screen, often 80 columns by 24 or 25 lines. Each character was displayed as a small [[dot matrix]], often about 8 [[pixel]]s wide, and a [[SBCS]] (single
For a number of practical and aesthetic reasons [[Han character]]s need to be square, approximately twice as wide as these fixed-width SBCS characters. As these were typically encoded in a [[DBCS]] (double
On the other hand, early Japanese computing used a single-byte code page called [[JIS X 0201]] for [[katakana]]. These would be rendered at the same width as the other single-byte characters, making them [[half-width kana]] characters rather than normally proportioned kana. Although the JIS X 0201 standard itself did not specify half-width display for katakana, this became the visually distinguishing feature in [[Shift JIS]] between the single-byte JIS X 0201 and double-byte [[JIS X 0208]] katakana. Some IBM code pages used a similar treatment for Korean jamo,<ref name="ibm933">{{cite web|url=http://demo.icu-project.org/icu-bin/convexp?conv=ibm-933|title=ICU Demonstration - Converter Explorer|author=|date=|website=demo.icu-project.org|accessdate=7 May 2018}}</ref> based on the [[KS C 5601#1974|N-byte Hangul code]] and its [[EBCDIC]] translation.
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==In Unicode==
{{see also|Halfwidth and Fullwidth Forms (Unicode block)}}
For compatibility with existing character sets that contained both half- and fullwidth versions of the same character, [[Unicode]] allocated a single block at U+FF00–FFEF containing the necessary "alternative width" characters. This includes a fullwidth version of all the [[ASCII]] characters and some non-ASCII punctuation such as the Yen sign, halfwidth versions of
Unicode assigns ''every'' code point an "East Asian width" [[Unicode character property|property]]. This may be:<ref name="uax11">{{cite web|url=https://unicode.org/reports/tr11/|title=Unicode® Standard Annex #11: East Asian Width|last1=Lunde|first1=Ken|authorlink=Ken Lunde|publisher=[[Unicode Consortium]]|date=2019-01-25}}</ref>
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== See also ==
* [[Han unification]]
* [[CJK Symbols and Punctuation (Unicode block)|East Asian punctuation]]
* [[Em size]] – full width forms
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