some multi-byte character encodings (character encodings that can handle more than 256 different characters) are also true extended ascii. that means all ascii characters are encoded with a single byte with the same value as ascii, and these values are not used anywhere else. they can be used in file formats where only ascii bytes are used for keywords and file format syntax, while bytes 0x80-0xff might be used for free text, including most programming languages, where language keywords, variable names, and function names must be in ascii, but string constants and comments can use non-ascii characters. this makes it much easier to introduce a multi-byte character set into existing systems that use extended ascii.