Some articles on fullwidth:

Halfwidth And Fullwidth Forms - In Unicode
... Halfwidth and Fullwidth Forms is the name of Unicode block U+FF00–FFEF, the last of the Basic Multilingual Plane excepting the short Specials block at U+FFF0–FFFF ... the characters of ASCII 21 to 7E as fullwidth forms, that is, a fixed width form used in CJK computing ... U+FF00 does not correspond to a fullwidth ASCII 20 (space character), since that role is already fulfilled by U+3000 "ideographic space." Range U+FF65–FFDC encodes halfwidth forms of Katakana and ...