Fullwidth form
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In CJK computing, graphic characters are traditionally classed into fullwidth (in Taiwan and Hong Kong: 全形; elsewhere: 全角) and halfwidth (in Taiwan and Hong Kong: 半形; elsewhere: 半角) characters. With fixed-width fonts (now called bi-width by Westerners), a halfwidth character occupies half the width of a fullwidth character, hence the name.
In the days of computer terminals and text mode computing, characters were normally laid out in a grid, often 80 columns by 24 or 25 lines. Each character would be displayed as a small dot matrix, often about 8 pixels wide, and an SBCS (single byte character set) is generally used to encode the characters.
For a number of practical and aesthetic reasons, Han characters would need to be twice as wide as these fixed-width SBCS characters. These "fullwidth characters" were typically encoded in a DBCS (double byte character set) because of a practical reason (compatibility with off-the-shelf software), though some systems may use a variable-width or some other form of multi-byte encoding.
In Unicode, if a certain grapheme can be represented as either a fullwidth character or a halfwidth character, it is said to have both a fullwidth form and a halfwidth form
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