Citation:
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 languages including C to mark the end of a string.
7 (bell, BEL, \a, ^G), which may cause the device receiving it to emit a warning of some kind (usually audible).