Halfwidth and fullwidth forms

In CJK (Chinese, Japanese, and Korean) computing, graphic characters are traditionally classed into fullwidth and halfwidth characters. Unlike monospaced fonts, a halfwidth character occupies half the width of a fullwidth character, hence the name.

Halfwidth and Fullwidth Forms is also the name of a Unicode block U+FF00–FFEF, provided so that older encodings containing both halfwidth and fullwidth characters can have lossless translation to and from Unicode.

Rationale

In the days of text 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 small dot matrix, often about 8 pixels wide, and an SBCS (single-byte character set) was generally used to encode characters of Western languages.

For aesthetic reasons and readability, it is preferable for Chinese characters to be approximately square-shaped, therefore twice as wide as these fixed-width SBCS characters. As these were typically encoded in a DBCS (double-byte character set), this also meant that their width on screen in a duospaced 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 called JIS X 0201 for katakana.