A universal character set standard that defines the characters of most major scripts used in the modern world. In 1993, ISO adopted Unicode version 1.1 as ISO/IEC 10646-1:1993. ISO/IEC 10646 has two formats: UCS-2 is a 2-byte fixed-width format, and UCS-4 is a 4-byte fixed-width format. There are three levels of implementation, all relating to support for composite characters: Level 1 requires no composite character support. Level 2 requires support for specific scripts (including most of the Unicode scripts such as Arabic and Thai). Level 3 requires unrestricted support for composite characters in all languages.