- Half-width kana
Half-width kana (半角カナ) is half of
fullwidth form. It refers to the katakanacharacter portion of the character set specified by JIS X 0201.
Although an official name is JIS X 0201 katakana, half-width kana is the commonly known name and this term will be used in this article.
ASCIIis defined as a 7-bit character setand has room for 128 characters. However, since this standard was designed for the United States, it does not contain characters and symbols (for example, the ¥ yencurrency symbol) needed for representation of Japanese. JIS X 0201was developed in 1969, and since computers at that time simply did not have the computational power and memory necessary to process the thousands of Kanji(Chinese-based) characters that exist in written Japanese, thereforeo as a simplification, Kanji characters were always represented by katakana.
Half-width kana were developed as "...the first Japanese characters encoded on computers because they are used for Japanese telegrams. As single-byte characters..." ref|Lunde1999_1
To make katakana fit into the area allowed, some compromises were made: the diacritical marks
Dakutenand Handakutenare treated as separate characters instead of being part of the preceding character. This led to the so-called "half-width kana" and these compromises still cause problems today for computer programs, apart from frequently being considered to be visually unattractive.
＼Trailing 4 bits→
↓Leading 4 bits
0 1 2 3 4 5 6 7 8 9 a b c d e f 0 1 2 3 4 5 6 7 8 9 a ｡ ｢ ｣ ､ ･ ｦ ｧ ｨ ｩ ｪ ｫ ｬ ｭ ｮ ｯ b ｰ ｱ ｲ ｳ ｴ ｵ ｶ ｷ ｸ ｹ ｺ ｻ ｼ ｽ ｾ ｿ c ﾀ ﾁ ﾂ ﾃ ﾄ ﾅ ﾆ ﾇ ﾈ ﾉ ﾊ ﾋ ﾌ ﾍ ﾎ ﾏ d ﾐ ﾑ ﾒ ﾓ ﾔ ﾕ ﾖ ﾗ ﾘ ﾙ ﾚ ﾛ ﾜ ﾝ ﾞ ﾟ e f
Half-width kana on the Internet
SMTPand NNTPprotocols (used to deliver e-mail and Usenet, respectively) were formerly only able to transmit 7-bits, it was then the convention to use ISO-2022-JPfor sending e-mail in Japanese.
Since half-width kana is not contained in ISO-2022-JP, half-width kana cannot be included in a message, but when half-width kana was accidentally included in a message, it can become garbled during transmission.
This is no longer such a problem since most e-mail servers today use
ESMTP, and hence 8-bit characters are acceptable. Alternatively, an encoding system such as Base64 can be used and specified in the message using MIME.
The problems that exists in e-mail do not exist with Web pages since
HTTPaccepts 8-bit characters.
A problem that does exist is that computer programs have difficulties whether to treat a character as
Shift JIS, EUC-JP, or UTF-7- hence character code information should be specified with a HTTP response header or a Meta tag.
Misunderstanding of JIS X 0201
In fact, JIS X 0201 katakana is not half-width katakana. The standard doesn't define character's width. It defines only the code representation of katakana characters. The term "half-width" is just the remains of the old devices that displayed single-byte characters in half-width (as compared with double-byte ones). In JIS X 0201 standard, katakana characters in its code chart are printed in normal width, not half-width.
However, the misunderstanding that the standard defines "half-width" characters is widespread. People who know the standard will often say "so-called half-width kana."
Halfwidth and Fullwidth Forms
#Note|Lunde1999_1 Lunde, Ken. CJKV Information Processing. 1st ed. O'Reilly, 1999. p. 144-145
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