Control Character

In

computing

and telecommunication, a control character or non-printing character (NPC) is a code point (a

number

) in a character set, that does not represent a written symbol. They are used as in-band signaling to cause effects other than the addition of a symbol to the text. All other characters are mainly printing, printable, or graphic characters, except perhaps for the "

space

" character (see ASCII printable characters).

All entries in the ASCII table below code 32 (technically the C0 control code set) are of this kind, including

CR and LF

used to separate lines of text. The code 127 (DEL) is also a control character. Extended ASCII sets defined by ISO 8859 added the codes 128 through 159 as control characters, this was primarily done so that if the high bit was stripped it would not change a printing character to a C0 control code, but there have been some assignments here, in particular

NEL

. This second set is called the C1 set.

These 65 control codes were carried over to

Unicode

. Unicode added more characters that could be considered controls, but it makes a distinction between these "Formatting characters" (such as the zero-width non-joiner), and the 65 control characters.

The Extended Binary Coded Decimal Interchange Code (EBCDIC) character set contains 65 control codes, including all of the ASCII control codes plus additional codes which are mostly used to control IBM peripherals.

History

Procedural signs in Morse code are a form of control character.

A form of control characters were introduced in the 1870 Baudot code: NUL and DEL.
The 1901 Murray code added the carriage return (CR) and line feed (LF), and other versions of the Baudot code included other control characters.

The bell character (BEL), which rang a bell to alert operators, was also an early teletype control character.

Control characters have also been called "format effectors".

In ASCII

The control characters in ASCII still in common use include:
* 0 ( null, NUL, \0, ^@), originally intended to be an ignored character, but now used by many

programming language

s including C to mark the end of a string.
* 7 (bell, BEL, \a, ^G), which may cause the device to emit a warning such as a bell or beep sound or the screen flashing.
* 8 (

backspace

, BS, \b, ^H), may overprint the previous character.
* 9 (

horizontal tab

, HT, \t, ^I), moves the printing position right to the next tab stop.
* 10 (

line feed

, LF, \n, ^J), moves the print head down one line, or to the left edge and down. Used as the end of line marker in most

UNIX systems

and variants.
* 11 (

vertical tab

, VT, \v, ^K), vertical tabulation.
* 12 (form feed, FF, \f, ^L), to cause a printer to eject paper to the top of the next page, or a video terminal to clear the screen.
* 13 (carriage return, CR, \r, ^M), moves the printing position to the start of the line, allowing overprinting. Used as the end of line marker in Classic Mac OS, OS-9, FLEX (and variants). A CR+LF pair is used by CP/M-80 and its derivatives including

DOS

and Windows