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What a Big5 code actually encodes

An individual Big5 code does not always represent a complete semantic unit. The Big5 codes of logograms are always logograms, but codes in the "graphical characters" section are not always complete "graphical characters". What Big5 encodes are particular graphical representations of characters or part of characters that happen to fit in the space taken by two monospaced ASCII characters. This is a property of double-byte character sets as normally used in CJK (Chinese, Japanese, and Korean) computing, and is not a unique problem of Big5.

(The above might need some explanation by putting it in historical perspective, as it is theoretically incorrect: Back when text mode personal computing was still the norm, characters were normally represented as single bytes and each character takes one position on the screen. There was therefore a practical reason to insist that double-byte characters must take up two positions on the screen, namely that off-the-shelf, American-made software would then be usable without modification in a DBCS-based system. If a character can take an arbitrary number of screen positions, software that assumes that one byte of text takes one screen position would produce incorrect output. Of course, if a computer never had to deal with the text screen, the manufacturer would not enforce this artificial restriction; the Apple Macintosh is an example. Nevertheless, the encoding itself must be designed so that it works correctly on text-screen-based systems.)

To illustrate this point, consider the Big5 code 0xa14b (…). To English speakers this looks like an ellipsis and the Unicode standard identifies it as such; however, in Chinese, the ellipsis consists of six dots that fit in the space of two Chinese characters (……), so in fact there is no Big5 code for the Chinese ellipsis, and the Big5 code 0xa14b just represents half of a Chinese ellipsis. It represents only half of an ellipsis because the whole ellipsis should take the space of two Chinese characters, and in many DBCS systems one DBCS character must take exactly the space of one Chinese character.

Characters encoded in Big5 do not always represent things that can be readily used in plain text files; an example is "citation mark" (0xa1ca, ﹋), which is, when used, required to be typeset under the title of literary works. Another example is the Suzhou numerals, which is a form of scientific notation that requires the number to be laid out in a 2-D form consisting of at least two rows.