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InCJK (Chinese, Japanese, and Korean) computing,graphic characters are traditionally classed intofullwidth[a] andhalfwidth[b] characters. Unlikemonospaced fonts, a halfwidth character occupies half the width of a fullwidth character, hence the name.
Halfwidth and Fullwidth Forms is also the name of aUnicode block U+FF00–FFEF, provided so that older encodings containing both halfwidth and fullwidth characters can have lossless translation to and from Unicode.
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In the days oftext 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 smalldot matrix, often about 8pixels wide, and anSBCS (single-byte character set) was generally used to encode characters of Western languages.
For aesthetic reasons and readability, it is preferable forChinese characters to be approximately square-shaped, therefore twice as wide as these fixed-width SBCS characters. As these were typically encoded in aDBCS (double-byte character set), this also meant that their width on screen in aduospaced font was proportional to their byte length. Some terminals and editing programs could not deal with double-byte characters starting at odd columns, only even ones (some could not even put double-byte and single-byte characters in the same line). So the DBCS sets generally included Roman characters and digits also, for use alongside the CJK characters in the same line.
On the other hand, early Japanese computing used a single-byte code page calledJIS X 0201 forkatakana. These would be rendered at the same width as the other single-byte characters, making themhalf-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 inShift JIS between the single-byte JIS X 0201 and double-byteJIS X 0208 katakana. Some IBM code pages used a similar treatment forKorean jamo,[1] based on theN-byte Hangul code and itsEBCDIC translation.
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 theASCII characters and some non-ASCII punctuation such as the Yen sign, halfwidth versions of katakana andhangul, and halfwidth versions of some other symbols such as circles. Only characters needed for lossless round trip to existing character sets were allocated, rather than (for instance) making a fullwidth version of every Latin accented character.
Unicode assignsevery code point an "East Asian width"property. This may be:[2]
| Abbreviation | Name | Description |
|---|---|---|
| W | Wide | Naturally wide character, e.g.Hiragana. |
| Na | Narrow | Naturally narrow character, e.g.ISO Basic Latin alphabet. |
| F | Fullwidth | Wide variant withcompatibility normalisation to naturally narrow character, e.g. fullwidth Latin script. |
| H | Halfwidth | Narrow variant withcompatibility normalisation to naturally wide character, e.g.half-width kana. Includes U+20A9 (₩) as an exception. |
| A | Ambiguous | Characters included in East Asian DBCS codes but also in European SBCS codes, e.g.Greek alphabet. Duospaced behaviour can consequently vary. |
| N | Neutral | Characters which do not appear in East Asian DBCS codes, e.g.Devanagari. |
Terminal emulators can use this property to decide whether a character should consume one or two "columns" when figuring out tabs and cursor position.
OpenType has thefwid,halt,hwid, andvhal feature tags to be used to reproduce fullwidth or halfwidth form of a character.CSS provides control over these features usingfont-variant-east-asian andfont-feature-settings properties.[3]
