As many keyboards and computer terminals once directly generated ASCII code, the choice of control-C overlapped with the ASCII end-of-text character. This character has a numerical value of three, as "C" is the third letter of the alphabet. It was chosen to cause an interrupt as it is otherwise unlikely to be part of a program's interactive interface. Many other control codes, such as control-D for the end-of-transmission character, do not generate signals and are occasionally used to control a program.
