Full-width characters occupy twice the horizontal space of half-width characters in monospaced (fixed-width) fonts. Japanese hiragana, katakana, kanji, and full-width alphanumeric characters are all classified as full-width.

Character counting behavior varies by platform. X (Twitter) counts both full-width and half-width characters as 1, while some systems count full-width as 2. CJK typography books explain the historical origins and modern usage of full-width characters.

In databases using UTF-8, full-width characters consume 3 bytes each. A VARCHAR(255) column can hold at most 85 full-width characters.

Web form validation often requires converting between full-width and half-width characters, especially for phone numbers and postal codes in Japanese applications. Web internationalization guides cover these conversion patterns.