Sorry to be late. Just some background information. On 2015/04/28 14:57, Makoto Kato wrote:
Although I read JIS X 4051, it doesn't define that half-width katakana and full-width katakana are differently.
I was on the committee that updated JIS X 4015 (mostly liaison/observer role). The chair of that committee was Prof. Shibano (芝野 耕司), who was also chair of the committee responsible for the Japanese character standards as well as chair of ISO/IEC JTC1 SC2.
I very well remember that he explained at one point that as far as the standards were concerned, full-width and half-width versions were considered one and the same character. In modern terms, the standards' view was that single-byte and double-byte encodings of these characters were just different "encoding forms" of one and the same abstract character.
This view is confirmed e.g. by the character names used in the 1997 version (confirmed 2002) of JIS X 0201, which are just "KATAKANA LETTER A",... Anybody interested can dig deeper, JIS X 0201 was just what was most easily accessible to me now.
The justification behind this is that they are linguistically not different at all, and that they were intended just as a fallback due to technology (memory, display resolution) limitations.
In practice, technical restrictions in early limitations (one byte == one (half-width) character cell) led to a typographic distinction. The fact that half-width Kana used less space was exploited in fixed-pitch screen design. That lead to a desire to keep the distinction when round-tripping via Unicode, and thus to different character names.
Regards, Martin.
