The C0 and C1 control code or control character sets define control codes for use in text by computer systems that use ASCII and derivatives of ASCII. The codes 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.

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C0 codes are the range 00_HEX–1F_HEX and the default C0 set was originally defined in ISO 646 (ASCII). C1 codes are the range 80_HEX–9F_HEX and the default C1 set was originally defined in ECMA-48 (harmonized later with ISO 6429). The ISO/IEC 2022 system of specifying control and graphic characters allows other C0 and C1 sets to be available for specialized applications, but they are rarely used.

ASCII defined 32 control characters, plus a necessary extra character for the DEL character, 7F_HEX or 01111111_BIN (needed to punch out all the holes on a paper tape and erase it).

This large number of codes was desirable at the time, as multi-byte controls would require implementation of a state machine in the terminal, which was very difficult with contemporary electronics and mechanical terminals.

Only a few codes have maintained their use: BEL, ESC, and the "Format Effector" (FE_n) characters BS, TAB, LF, VT, FF, and CR. Others are unused or have acquired different meanings such as NUL being the C string terminator. Some data transfer protocols such as ANPA-1312, Kermit, and XMODEM do make extensive use of SOH, STX, ETX, EOT, ACK, NAK and SYN for purposes approximating their original definitions; and some file formats use the "Information Separators" (IS_n) such as the Unix info format^[1] and Python's splitlines string method.^[2]

The names of some codes were changed in ISO 6429:1992 (or ECMA-48:1991) to be neutral with respect to writing direction. The abbreviations used were not changed, as the standard had already specified that those would remain unchanged when the standard is translated to other languages. In this table both new and old names are shown for the renamed controls (the old name is the one matching the abbreviation).

In 1973, ECMA-35 and ISO 2022^[17] attempted to define a method so an 8-bit "extended ASCII" code could be converted to a corresponding 7-bit code, and vice versa.^[18] In a 7-bit environment, the Shift Out (SO) would change the meaning of the 96 bytes 0x20 through 0x7F^{[lower-alpha 1][20]} (i.e. all but the C0 control codes), to be the characters that an 8-bit environment would print if it used the same code with the high bit set. This meant that the range 0x80 through 0x9F could not be printed in a 7-bit environment,^[18] thus it was decided that no alternative character set could use them, and that these codes should be additional control codes, which become known as the C1 control codes. To allow a 7-bit environment to use these new controls, the sequences ESC @ through ESC _ were to be considered equivalent.^[18] The later ISO 8859 standards abandoned support for 7-bit codes, but preserved this range of control characters.

The first C1 control code set to be registered for use with ISO 2022 was DIN 31626,^[21] a specialised set for bibliographic use which was registered in 1979.^[22]

The more common general-use ISO/IEC 6429 set was registered in 1983,^[23] although the ECMA-48 specification upon which it was based had been first published in 1976^[24] and JIS X 0211 (formerly JIS C 6323).^[25] Symbolic names defined by RFC 1345 and early drafts of ISO 10646, but not in ISO/IEC 6429 (PAD, HOP and SGC) are also used.^[8][26]

Except for SS2 and SS3 in EUC-JP text, and NEL in text transcoded from EBCDIC, the 8-bit forms of these codes were almost never used. CSI, DCS and OSC are used to control text terminals and terminal emulators, but almost always by using their 7-bit escape code representations. Nowadays if these codes are encountered it is far more likely they are intended to be printing characters from that position of Windows-1252 or Mac OS Roman.

The ISO/IEC 2022 (ECMA-35) extension mechanism allowed escape sequences to change the C0 and C1 sets. The standard C0 control character set shown above is chosen with the sequence ESC ! @ and the above C1 set chosen with the sequence ESC " C.^[23]

Several official and unofficial alternatives have been defined, but this is pretty much obsolete. Most were forced to retain a good deal of compatibility with the ASCII controls for interoperability. The standard makes ESC,^[34][35] SP and DEL^{[lower-alpha 1]} "fixed" coded characters, which are available in their ASCII locations in all encodings that conform to the standard.^[37] It also specifies that if a C0 set included transmission control (TC_n) codes, they must be encoded at their ASCII locations^[34] and could not be put in a C1 set,^[38] and any new transmission controls must be in a C1 set.^[34]

Unicode inherits its first 256 code points from ISO 8859-1, hence also the 65 code points described above, giving them the general category Cc (control). These are:

• U+0000–U+001F (C0 controls) and U+007F (DEL) assigned to the Basic Latin block, and
• U+0080–U+009F (C1 controls) assigned to the Latin-1 Supplement block.

Unicode only specifies semantics for the C0 format controls HT, LF, VT, FF, and CR, (note BS is missing); the C0 information separators FS, GS, RS, US (and SP); and the C1 control NEL.^[49] The rest of the codes are transparent to Unicode and their meanings are left to higher-level protocols, with ISO/IEC 6429 suggested as a default.^[49]

Unicode includes many additional format effector characters besides these, such as marks, embeds, isolates and pops for explicit bidirectional formatting, and the zero-width joiner and non-joiner for controlling ligature use. However these are given the general category Cf (format) rather than Cc.

• Control Pictures
• ANSI escape code