Most character encodings, in addition to representing printable characters, may also represent additional information about the text, such as the position of a cursor, an instruction to start a new line, or a message that the text has been received. The C0 and C1 control code sets define control codes for use in text by computer systems that use the ISO/IEC 2022 system of specifying control and graphic characters. The C0 set defines codes in the range 00HEX–1FHEX and the C1 set defines codes in the range 80HEX–9FHEX. The default C0 set was originally defined in ISO 646 (ASCII), while the default C1 set was originally defined in ECMA-48 (harmonized later with ISO 6429). While other C0 and C1 sets are available for specialized applications, they are rarely used.
While the C1 control characters are used in conjunction with the ISO/IEC 8859 series of graphical character sets among others, and are integrated into Unicode, they are rarely used directly, except on specific platforms such as OpenVMS. When they turn up in documents, Web pages, e-mail messages, etc., which are ostensibly in an ISO-8859-n encoding, their code positions generally refer instead to the characters at that position in a proprietary, system-specific encoding such as Windows-1252 or the Apple Macintosh ("MacRoman") character set that use the codes provided for representation of the C1 set with a single 8-bit byte to instead provide additional graphic characters, though this is technically invalid under the ISO encodings. The byte values for the C1 set are also used by the UTF-8 encoding of Unicode. Thus the corresponding control functions are more commonly accessed using the equivalent two byte escape sequence intended for use with systems that have only 7-bit bytes.
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