Halfwidth and fullwidth forms

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.

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.

On the other hand, early Japanese computing used a single-byte code page called JIS X 0201 for katakana.

This may be:[2] Terminal emulators can use this property to decide whether a character should consume one or two "columns" when figuring out tabs and cursor position.

OpenType has the fwid, halt, hwid, and vhal feature tags to be used to reproduce fullwidth or halfwidth form of a character.

A command prompt ( cmd.exe ) with Korean localisation, showing halfwidth and fullwidth characters
Characters which appear in both JIS X 0201 (single byte) and JIS X 0208 / JIS X 0213 (double byte) have both a halfwidth and a fullwidth form in Shift JIS .